feat: implement app version auto-update mechanism

[collinsezedike]

What this does

Closes #356.

Adds an in-app update system that checks for new versions on startup and prompts users to install with one tap, satisfying all acceptance criteria:

• Version check on app start - AppUpdateService.checkForUpdate() runs on mount via UpdateChecker in the root layout. A 24-hour interval guard (shouldCheck()) prevents redundant calls on every launch.
• Update available notification - UpdateNotificationModal renders an accessible modal with the update description and optional release notes. Mandatory updates block dismissal.
• One-click OTA update - For JS-only bundle changes, the modal's "Install Now" button calls fetchUpdateAsync() then reloadAsync() via expo-updates (dynamically imported, so the app won't crash before the package is added to a native build). Native/binary changes fall back to a "Open App Store / Open Play Store" path using the store URLs already defined in app.json.
• Update adoption tracking - mobileAnalyticsService.trackEvent fires on every stage: check started, update available/not available, download started/completed/failed, update applied, dismissed, and store redirect. Nine new UPDATE_* events added to AnalyticsEvent.
• End-to-end test coverage - appUpdateService.test.ts (10 cases) and useAppUpdate.test.ts (9 cases) cover all state transitions, error paths, platform branching, and the mount-time auto-check.

Key files

File	Role
src/services/appUpdateService.ts	Core service: OTA check, download/apply, store redirect, analytics
src/hooks/useAppUpdate.ts	React hook wrapping the service with loading/error state
src/components/common/UpdateNotificationModal.tsx	Accessible update prompt UI
src/__mocks__/expo-updates.ts	Jest mock for expo-updates
app/_layout.tsx	UpdateChecker component wired into the provider tree
app.json	expo-updates plugin, updates config, runtimeVersion policy
src/utils/trackingEvents.ts	Nine new UPDATE_* analytics events

Notes

• expo-updates is not yet in package.json (package installation was deferred). It needs npx expo install expo-updates before the next native build. The dynamic import means existing JS bundle runs are unaffected until then.
• The src/services/index.ts file had stale merge-conflict prose as its content; that was corrected as part of this PR.

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