fix(ai): improve AI settings UX with validation and smart fallbacks

Backend:
- Add smart provider fallback when selected model's provider isn't configured
- Automatically switch to a model from a configured provider instead of failing
- Log warning when fallback occurs for visibility

Frontend (AISettings.tsx):
- Add helper functions to check if model's provider is configured
- Group model dropdown: configured providers first, unconfigured marked with ⚠️
- Add inline warning when selecting model from unconfigured provider
- Validate on save that model's provider is configured (or being added)
- Warn before clearing last configured provider (would disable AI)
- Warn before clearing provider that current model uses
- Add patrol interval validation (must be 0 or >= 10 minutes)
- Show red border + inline error for invalid patrol intervals 1-9
- Update patrol interval hint: '(0=off, 10+ to enable)'

These changes prevent confusing '500 Internal Server Error' and
'AI is not enabled or configured' errors when model/provider mismatch.

frontend-modern/src/components/Settings/UpdatesSettingsPanel.tsx

@@ -591,6 +591,7 @@ sudo tar -xzf pulse-${props.updateInfo()?.latestVersion}-linux-amd64.tar.gz -C /
                     <label class="relative inline-flex items-center cursor-pointer">
                       <input
                         type="checkbox"
+                        data-testid="updates-auto-check-toggle"
                         checked={props.autoUpdateEnabled()}
                         onChange={(e) => {
                           props.setAutoUpdateEnabled(e.currentTarget.checked);