Closes#918.
The renderer's keydown handler required event.metaKey and explicitly
rejected event.ctrlKey, so every shortcut was dead outside macOS: on
Windows and Linux metaKey is the Super key, which the OS shell takes.
Navigation (1-8), Settings (,) and Refresh (R) all did nothing. The
sidebar and footer hints also hardcoded the Cmd glyph, so a Windows
user was shown chords that could not fire.
Add app/renderer/lib/platform.ts as the single source of truth for
platform-aware shortcuts, reading the platform the preload already
exposes (window.codeburn.platform) with a user-agent fallback for the
non-Electron cases. isModifierChord accepts Cmd-without-Ctrl on darwin
and Ctrl-without-Cmd elsewhere; altKey stays rejected on both, because
AltGr on European Windows layouts arrives as Ctrl+Alt and must not
hijack a typed character. Every visible shortcut label now resolves
through shortcutLabel() at render time, so the sidebar shows Ctrl+1
where macOS shows the Cmd glyph.
The mac chord condition is unchanged: the old guard admitted
metaKey && !altKey && !ctrlKey && !shiftKey, and the new one admits
exactly the same set on darwin.
The Electron application menu is deliberately left alone. It ships no
reload/forceReload role and no CmdOrCtrl+R accelerator, which is what
leaves Ctrl+R free for the renderer to handle on Windows.
Also corrects the Settings navigation hint, which read 1-7 while the
sidebar has eight numbered destinations.
Tests cover both platforms for labels and dispatch, including the
negatives: Meta on win32, Ctrl on darwin, and the Ctrl+Alt AltGr shape.
