windows: cfg-gate the tray badge so the Linux build has no dead code

The spend badge is a second tray icon carrying the number as its bitmap, which
only the Tauri tray backend provides; Linux runs its own SNI tray and has no
equivalent. The module was compiled there anyway, so every item in it - and the
two tray ids in lib.rs - tripped dead_code under clippy -D warnings on the
ubuntu leg.

- `mod tray_badge` and TRAY_ID / BADGE_TRAY_ID are now cfg(not(linux)), so the
  code is absent on Linux rather than present and unused.
- `set_tray_badge` reports the badge as unsupported on Linux instead of
  returning a success that never happened.
- The frontend hides the control wherever it is unsupported, behind
  TRAY_BADGE_SUPPORTED in lib/platform.ts - one constant, three call sites
  (the settings row, the footer menu item, and the effect that would otherwise
  invoke the command).
- AppState's `linux_tray` field is dropped: it was written and never read, which
  is the same lint one file over. init_tray_linux already owns the handle.

Checked rather than reasoned: `cargo clippy --all-targets -- -D warnings` passes
on macOS, and a scratch copy of the crate with the linux/macos cfg arms swapped
(so a normal macOS clippy selects everything a Linux build would keep, and drops
everything it would drop) also passes. Reinstating the ungated `mod tray_badge`
in that copy reproduces the exact CI failure, so the check is real. It stubs
tray_linux.rs, whose ksni and png deps do not build here - that file is still
only covered by CI's ubuntu leg.

windows/DEVELOPMENT.md

@@ -8,6 +8,12 @@ Linux (ksni / AppIndicator) support is compiled and kept working for dev, but it
 **experimental and unreleased** - Linux users should use the GNOME extension in `../gnome/`.
 The releases this repo cuts from here are Windows only.
 
+Not everything crosses over: the spend badge is a second tray icon carrying the number as its
+bitmap, which only the Windows notification area provides. `tray_badge` is compiled out on
+Linux, the `set_tray_badge` command reports it as unsupported there, and the frontend hides
+the control behind `TRAY_BADGE_SUPPORTED` in `src/lib/platform.ts`. Anything else that is
+Windows-only must be cfg-gated the same way, or the ubuntu leg of CI fails on dead code.
+
 ## Architecture
 
 ```

windows/src-tauri/src/lib.rs

@@ -3,6 +3,10 @@ mod cli;
 mod config;
 mod fx;
 mod plan;
+/// The spend-in-the-tray badge is a second tray icon, which only the Tauri tray backend
+/// provides; Linux runs its own SNI tray (`tray_linux`) and has no equivalent, so the
+/// whole module is compiled out there rather than sitting unused.
+#[cfg(not(target_os = "linux"))]
 mod tray_badge;
 #[cfg(target_os = "linux")]
 mod tray_linux;
@@ -26,9 +30,11 @@ use crate::cli::CodeburnCli;
 use crate::config::CurrencyConfig;
 use crate::fx::FxCache;
 
+#[cfg(not(target_os = "linux"))]
 const TRAY_ID: &str = "codeburn-tray";
 /// Second tray icon that carries today's spend as text, sitting next to the logo. The
-/// closest Windows and Linux panels get to the macOS menubar title.
+/// closest the Windows notification area gets to the macOS menubar title.
+#[cfg(not(target_os = "linux"))]
 const BADGE_TRAY_ID: &str = "codeburn-badge";
 const POPOVER_LABEL: &str = "popover";
 
@@ -41,8 +47,6 @@ pub struct AppState {
     pub config: Mutex<CurrencyConfig>,
     pub fx: FxCache,
     pub plan: plan::PlanClient,
-    #[cfg(target_os = "linux")]
-    pub linux_tray: tray_linux::LinuxTrayHandle,
 }
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
@@ -51,24 +55,18 @@ pub fn run() {
         .plugin(tauri_plugin_shell::init())
         .plugin(tauri_plugin_opener::init())
         .setup(|app| {
-            #[cfg(target_os = "linux")]
-            let linux_tray = tray_linux::LinuxTrayHandle::empty();
-
-            let state = AppState {
+            app.manage(AppState {
                 cli: Mutex::new(CodeburnCli::resolve()),
                 config: Mutex::new(CurrencyConfig::load_or_default()),
                 fx: FxCache::new(),
                 plan: plan::PlanClient::new(),
-                #[cfg(target_os = "linux")]
-                linux_tray: linux_tray.clone(),
-            };
-            app.manage(state);
+            });
 
             #[cfg(not(target_os = "linux"))]
             build_tray_tauri(app.handle())?;
 
             #[cfg(target_os = "linux")]
-            init_tray_linux(app.handle().clone(), linux_tray);
+            init_tray_linux(app.handle().clone(), tray_linux::LinuxTrayHandle::empty());
 
             if let Some(window) = app.get_webview_window(POPOVER_LABEL) {
                 let _ = window.hide();
@@ -466,6 +464,14 @@ mod commands {
     /// `text` is a short spend string ("$87", "142", "1.2K"); `None` hides the badge icon.
     #[tauri::command]
     pub fn set_tray_badge(app: AppHandle, text: Option<String>) -> Result<(), String> {
+        #[cfg(target_os = "linux")]
+        {
+            // Unreachable from the UI: the frontend hides the control wherever the badge is
+            // unsupported (lib/platform.ts). Saying so beats reporting a success that never
+            // happened.
+            let _ = (app, text);
+            Err("the tray spend badge needs a second tray icon, which the Linux SNI tray does not provide".to_string())
+        }
         #[cfg(not(target_os = "linux"))]
         {
             let Some(badge) = app.tray_by_id(super::BADGE_TRAY_ID) else {
@@ -487,12 +493,8 @@ mod commands {
                     badge.set_visible(false).map_err(|e| e.to_string())?;
                 }
             }
+            Ok(())
         }
-        #[cfg(target_os = "linux")]
-        {
-            let _ = (app, text);
-        }
-        Ok(())
     }
 
     #[tauri::command]

windows/src/App.tsx

@@ -8,6 +8,7 @@ import { USD, formatCurrency, trayBadgeText } from './lib/currency'
 import { PayloadCache } from './lib/cache'
 import { relativePast } from './lib/dates'
 import { applyTheme, currentTheme, readSetting, writeSetting } from './lib/settings'
+import { TRAY_BADGE_SUPPORTED } from './lib/platform'
 import { AgentTabStrip, detectedProviders } from './components/AgentTabStrip'
 import type { Provider } from './components/AgentTabStrip'
 import { ModelsSection } from './components/ModelsSection'
@@ -65,7 +66,7 @@ export function App() {
   const [version, setVersion] = useState('')
   const [lastUpdated, setLastUpdated] = useState<Date | null>(null)
   const [theme, setTheme] = useState(() => currentTheme())
-  const [trayBadge, setTrayBadge] = useState(() => readSetting('trayBadge') !== 'off')
+  const [trayBadge, setTrayBadge] = useState(() => TRAY_BADGE_SUPPORTED && readSetting('trayBadge') !== 'off')
   const [showSettings, setShowSettings] = useState(false)
   // The window starts hidden and is shown by a tray click, which emits `codeburn://shown`.
   const [popoverVisible, setPopoverVisible] = useState(false)
@@ -216,6 +217,7 @@ export function App() {
   }, [todayCost, currency])
 
   useEffect(() => {
+    if (!TRAY_BADGE_SUPPORTED) return
     const text = trayBadge && todayCost !== null ? trayBadgeText(todayCost, currency) : null
     invoke('set_tray_badge', { text }).catch(err => setError(`Tray badge: ${String(err)}`))
   }, [todayCost, currency, trayBadge])

windows/src/components/FooterBar.tsx

@@ -1,5 +1,6 @@
 import type { CurrencyState } from '../lib/currency'
 import { CURRENCY_CODES } from '../lib/currency'
+import { TRAY_BADGE_SUPPORTED } from '../lib/platform'
 import { DropMenu } from './DropMenu'
 import { CoinIcon, DownloadIcon, EllipsisIcon, RefreshIcon, TerminalIcon } from './Icons'
 
@@ -64,7 +65,9 @@ export function FooterBar({
         className="dropmenu-more"
         items={[
           { id: 'settings', label: settingsOpen ? 'Back to overview' : 'Settings…' },
-          { id: 'badge', label: "Show today's cost in tray", checked: trayBadge, separatorBefore: true },
+          ...(TRAY_BADGE_SUPPORTED
+            ? [{ id: 'badge', label: "Show today's cost in tray", checked: trayBadge, separatorBefore: true }]
+            : []),
           { id: 'theme', label: themeLabel },
           { id: 'quit', label: 'Quit CodeBurn', separatorBefore: true },
         ]}

windows/src/components/SettingsPanel.tsx

@@ -3,7 +3,7 @@ import { invoke } from '@tauri-apps/api/core'
 import { openUrl } from '@tauri-apps/plugin-opener'
 import type { CurrencyState } from '../lib/currency'
 import { CURRENCY_CODES } from '../lib/currency'
-import { homePath } from '../lib/platform'
+import { homePath, TRAY_BADGE_SUPPORTED } from '../lib/platform'
 import type { CliStatus } from './SetupState'
 import { DropMenu } from './DropMenu'
 import { ChevronDown, ChevronRight } from './Icons'
@@ -70,9 +70,11 @@ export function SettingsPanel({
           <Toggle on={loginItem === true} disabled={loginItem === null} onToggle={toggleLogin} />
         </Row>
         {loginError && <div className="settings-error">{loginError}</div>}
-        <Row label="Show today's cost in the tray" hint="A second tray icon carrying the number, next to the logo.">
-          <Toggle on={trayBadge} onToggle={() => onTrayBadge(!trayBadge)} />
-        </Row>
+        {TRAY_BADGE_SUPPORTED && (
+          <Row label="Show today's cost in the tray" hint="A second tray icon carrying the number, next to the logo.">
+            <Toggle on={trayBadge} onToggle={() => onTrayBadge(!trayBadge)} />
+          </Row>
+        )}
       </div>
 
       <div className="settings-group">

windows/src/lib/platform.ts

@@ -8,3 +8,9 @@ const SEP = IS_WINDOWS ? '\\' : '/'
 export function homePath(...parts: string[]): string {
   return [HOME, ...parts].join(SEP)
 }
+
+/// Today's spend in the tray is a second tray icon carrying the number as its bitmap. Only
+/// the Windows notification area gives us one; the Linux SNI tray has no equivalent, and
+/// macOS ships the Swift menubar instead. Where this is false the control is hidden and the
+/// Rust command is never called.
+export const TRAY_BADGE_SUPPORTED = IS_WINDOWS