feat(desktop): improve auto-updater UX and add global update indicator

[wesbillman]

Summary

• Restyled the Settings → Updates section to match the design system (semantic tokens, <Button>, <Badge> — no more hardcoded zinc/blue colors)
• Extracted updater state machine into useUpdater hook + UpdaterProvider context (single instance shared between settings panel and top bar)
• Added UpdateIndicator in the top nav bar: pulsing dot when update available, green dot when ready to restart — clicking opens Settings → Updates
• Passive Sonner toast notification when an update is detected (deduped, no auto-open)
• Dev build graceful fallback: when the updater plugin isn't loaded, silently stays in idle state instead of showing an error
• Wrapped relaunch() to surface errors in UI instead of unhandled promise rejections

Files changed

File	What
desktop/src/features/settings/hooks/use-updater.ts	NEW — updater state machine hook
desktop/src/features/settings/hooks/UpdaterProvider.tsx	NEW — React context for single updater instance
desktop/src/features/settings/UpdateIndicator.tsx	NEW — top-bar update indicator component
desktop/src/features/settings/UpdateChecker.tsx	Rewritten as pure presentational component
desktop/src/app/AppShell.tsx	Wraps app with UpdaterProvider, adds UpdateIndicator

Notable design decisions

• Single context instance — both the top-bar indicator and the settings panel share one useUpdater() via context. No split-brain, no duplicate API calls.
• Passive notifications only — toast is informational with no action button. Settings only opens on explicit user interaction (clicking top-bar icon or navigating manually). No interrupting the user on app launch.
• Dev-safe — catches plugin-not-found errors from Tauri and falls back to idle state, so dev builds without sprout_updater_enabled work cleanly.

Test plan

• Build a release with updater enabled, verify toast appears when update is available
• Verify top-bar Download icon appears with pulsing dot when update available
• Verify top-bar RefreshCw icon appears with green dot after install completes
• Click top-bar icon → Settings opens to Updates tab
• Verify Settings → Updates section matches other settings sections visually
• Dev build: confirm no error state on launch, idle state shows correctly
• Click "Download & Install" → verify downloading/installing/ready states transition correctly
• Error scenario: verify error message displays and Retry button works

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