Add notification and battery optimization permission checks at launch

[keithvassallomt]

Summary

• Adds recurring permission checks when the app opens for notification permission (Android 13+) and battery optimization exemption
• Shows a dialog explaining why each permission is needed, with "Enable" and "Not now" buttons
• Includes a "Don't ask again" checkbox that permanently suppresses the prompt (stored in SharedPreferences)
• Adds a "Reset permission reminders" option in Settings > Notifications to re-enable suppressed prompts
• Permission checks are skipped while onboarding tips are still active, so new users complete the tutorial first
• Dialogs are shown one at a time in onResume, so they re-check when the user returns from system settings

Note: This PR is a prerequisite for a follow-up PR that adds broadcast intent support for external app control of timers (see the broadcast-intents branch). The battery optimization exemption in particular is needed for broadcast receivers to start foreground services on Android 12+.

Test plan

• Revoke notification permission on Android 13+ emulator — notification dialog appears on launch
• Tap "Enable" — opens Android notification settings for the app
• Grant permission, return to app — battery optimization dialog appears next
• Tap "Enable" on battery dialog — system battery optimization prompt appears
• Grant exemption, return to app — no more dialogs
• Revoke permission again, check "Don't ask again" + tap "Not now" — dialog does not reappear
• Go to Settings > Notifications > "Reset permission reminders" — snackbar confirms reset
• Reopen app — suppressed dialogs reappear
• Fresh install — verify onboarding tips show first, permission dialogs only appear after onboarding completes
• Run ./gradlew test — no regressions
• Test all of the above on an actual device (tested on Pixel 10 Pro Fold).

[DeweyReed]

Thank you for your contribution!

Asking for permissions right after the app launches is a terrible user experience. Ideally, apps trigger permission requests when the user performs some actions. For example, the app requests the notification permission only when the user starts a timer.

The battery optimization exemption isn't enough on many smartphones, which is why the app has a whitelist guideline.

What does the broadcast intent support look like?

[keithvassallomt]

Thank you for the feedback — you're right that prompting for permissions at launch is poor UX, and the whitelist guideline already handles battery optimization better than a simple exemption request.

I'm going to close this PR. The broadcast intent support is in a separate branch, which I'll submit as a new PR shortly. It adds a public broadcast API for external timer control (create, start, pause, resume, reset, dismiss, list) — useful for voice assistant integration.

The only permission-related change in the new PR is a settings entry for the full-screen intent permission (Android 14+), which auto-hides once granted. This one doesn't have a natural in-flow prompt point since users won't know it's missing until a timer fires without the full-screen overlay.