Add automatic streak-saver notification

[bootuz]

Summary

• Adds a third reminder — an automatic streak-saver — that fires in the evening (fixed 20:00) only when the streak is at risk: currentStreak > 0 && studiedToday == 0 && freezeTokens == 0.
• No toggle / no settings / no UI — it's always armed; the OS notification permission (and the "reminders" channel) is the only off switch. Integrates with freezes: it won't nag a streak that a freeze will protect.
• Reuses the existing self-rescheduling ReminderWorker + "reminders" channel (no new infra).

Implementation

• ReminderContent.kt: new ReminderType.StreakSaver + STREAK_SAVER_HOUR/MINUTE constants; the pure reminderContent(...) gains currentStreak/freezeTokens params + a StreakSaver branch.
• ReminderWorker.kt: injects StreakProvider + StreakStateRepository, feeds the live streak + freeze count into the decision; scheduleFor returns always-enabled at the fixed time.
• AzriApplication.ensureReminders(): arms the streak-saver unconditionally on launch.

Test Plan

• ./gradlew :app:testDebugUnitTest — green (4 new ReminderContentTest cases: fires at-risk+unprotected; skips when studied-today / no-streak / freeze-protected)
• ./gradlew :app:assembleDebug — debug APK builds
• Manual (emulator): streak alive + 0 freezes + not studied today → at ~20:00 a "Keep your streak alive" notification appears; with a freeze available, or after studying today, nothing posts

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