Add streak freeze and repair

[bootuz]

Summary

• Freeze: earn 1 streak freeze per 7 consecutive study days (cap 2); a missed day is auto-covered by a freeze so the streak survives. Free for everyone, no in-app currency.
• Repair: if a streak breaks from a single recent missed day, a free one-tap "Repair" restores it — available after studying today, once per 30 days.
• Synced via Firestore (last-write-wins), so the streak number stays consistent across devices.
• The pure StreakCalculator is unchanged: a frozen day is fed to it as just another active day. All the new statefulness lives in one reconciler.

How it works

• StreakReconciler (pure): decides which missed days become frozen, earns freezes (idempotent, capped), and computes repair eligibility.
• StreakState persisted in Room (single-row table, migration v2→v3) and synced as a per-user Firestore doc via the existing SyncManager (LWW).
• StreakStateManager ties the reconciler to the repos; reconcile runs on app foreground + session finish (never inside a Flow).
• StreakProvider unions frozen days into the streak; the Today header shows a ❄️+count chip and a "Repair your N-day streak" banner; the session summary surfaces freeze count + repair.

Test Plan

• ./gradlew :app:testDebugUnitTest — green (new: StreakReconcilerTest 9, StreakStateManagerTest, StreakStateRepositoryTest, SyncManagerTest streak LWW, StreakProviderTest union, StudyQueueViewModelTest freeze/repair exposure)
• ./gradlew :app:assembleDebug — debug APK builds
• ./gradlew :app:compileDebugAndroidTestKotlin — androidTest compiles
• Migration MIGRATION_2_3 SQL verified character-for-character against Room's generated schema
• Manual (emulator): build a streak, miss a day → a freeze is consumed and 🔥 survives with ❄️ shown; with no freeze, the streak breaks → after studying today a "Repair" banner appears → tap restores it

🤖 Generated with Claude Code