fix: epoch-millis DB converter + biometric from onResume

Sync change detection:
- DbConverters was using epochSecond but comparisons used epochMilli —
  every file appeared modified on every scan, causing full re-sync each time
- DB migration 2→3 clears sync_file_states (all stored timestamps wrong)
- First sync after upgrade re-learns state; subsequent syncs skip unchanged files

Biometric:
- Move prompt trigger from LaunchedEffect to onResume() — guarantees
  the activity is in RESUMED state when authenticate() is called
- Add bestAuthenticators(): tries BIOMETRIC_STRONG|DEVICE_CREDENTIAL first,
  falls back to BIOMETRIC_WEAK|DEVICE_CREDENTIAL for side-sensor phones
- canAuthenticate() now accepts either strong or weak+credential
- onAuthenticationError always shows Unlock button (no infinite retry loop)
- isLocked/showRetry are Activity-level state, no need for Compose remember

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/di/AppModule.kt

@@ -21,9 +21,8 @@ object AppModule {
     @Provides @Singleton
     fun provideDatabase(@ApplicationContext ctx: Context): SyncDatabase =
         Room.databaseBuilder(ctx, SyncDatabase::class.java, "syncflow.db")
-            // Only fall back to destructive migration for very old dev builds (v1).
-            // All future version bumps must include a proper Migration object.
             .fallbackToDestructiveMigrationFrom(1)
+            .addMigrations(SyncDatabase.MIGRATION_2_3)
             .build()
 
     @Provides fun provideCloudAccountDao(db: SyncDatabase): CloudAccountDao = db.cloudAccountDao()