fix: correct Room migration - use DEFAULT '' to match entity schema

Migration 1→2 had DEFAULT NULL but @ColumnInfo(defaultValue="") expects
DEFAULT ''. Room schema validation failed on startup.

Since SQLite can't alter column defaults, add migration 2→3 that recreates
the alarms table with the correct DEFAULT '' for the date column.

app/src/main/java/com/example/helios_alarm_clock/data/AlarmDatabase.kt

@@ -5,16 +5,41 @@ import androidx.room.RoomDatabase
 import androidx.room.migration.Migration
 import androidx.sqlite.db.SupportSQLiteDatabase
 
-@Database(entities = [AlarmEntity::class], version = 2, exportSchema = true)
+@Database(entities = [AlarmEntity::class], version = 3, exportSchema = true)
 abstract class AlarmDatabase : RoomDatabase() {
     abstract fun alarmDao(): AlarmDao
 
     companion object {
         const val NAME = "helios_alarms.db"
 
+        // Fresh install path: add date column with correct default
         val MIGRATION_1_2 = object : Migration(1, 2) {
             override fun migrate(db: SupportSQLiteDatabase) {
-                db.execSQL("ALTER TABLE alarms ADD COLUMN date TEXT DEFAULT NULL")
+                db.execSQL("ALTER TABLE alarms ADD COLUMN date TEXT DEFAULT ''")
+            }
+        }
+
+        // Fix path: first install had DEFAULT NULL instead of DEFAULT ''
+        // SQLite can't alter column defaults, so recreate the table
+        val MIGRATION_2_3 = object : Migration(2, 3) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL("""
+                    CREATE TABLE alarms_new (
+                        id TEXT NOT NULL PRIMARY KEY,
+                        hour INTEGER NOT NULL,
+                        minute INTEGER NOT NULL,
+                        label TEXT NOT NULL,
+                        triggerTimeMillis INTEGER NOT NULL,
+                        date TEXT DEFAULT ''
+                    )
+                """.trimIndent())
+                db.execSQL("""
+                    INSERT INTO alarms_new (id, hour, minute, label, triggerTimeMillis, date)
+                    SELECT id, hour, minute, label, triggerTimeMillis, COALESCE(date, '')
+                    FROM alarms
+                """.trimIndent())
+                db.execSQL("DROP TABLE alarms")
+                db.execSQL("ALTER TABLE alarms_new RENAME TO alarms")
             }
         }
     }

app/src/main/java/com/example/helios_alarm_clock/di/AppModule.kt

@@ -23,7 +23,7 @@ object AppModule {
             AlarmDatabase::class.java,
             AlarmDatabase.NAME
         )
-            .addMigrations(AlarmDatabase.MIGRATION_1_2)
+            .addMigrations(AlarmDatabase.MIGRATION_1_2, AlarmDatabase.MIGRATION_2_3)
             .build()
 
     @Provides