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/data/db/SyncDatabase.kt

@@ -3,6 +3,8 @@ package com.syncflow.data.db
 import androidx.room.Database
 import androidx.room.RoomDatabase
 import androidx.room.TypeConverters
+import androidx.room.migration.Migration
+import androidx.sqlite.db.SupportSQLiteDatabase
 import com.syncflow.data.db.entities.*
 
 @Database(
@@ -13,11 +15,21 @@ import com.syncflow.data.db.entities.*
         SyncConflictEntity::class,
         SyncEventEntity::class,
     ],
-    version = 2,
+    version = 3,
     exportSchema = true,
 )
 @TypeConverters(DbConverters::class)
 abstract class SyncDatabase : RoomDatabase() {
+
+    companion object {
+        // Wipe file states: timestamps were stored as epoch-seconds, now epoch-millis.
+        // All previously saved states are wrong so we drop and re-learn on next sync.
+        val MIGRATION_2_3 = object : Migration(2, 3) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL("DELETE FROM sync_file_states")
+            }
+        }
+    }
     abstract fun cloudAccountDao(): CloudAccountDao
     abstract fun syncPairDao(): SyncPairDao
     abstract fun syncFileStateDao(): SyncFileStateDao