feat: add brand asset upload with PNG conversion and improve config validation

- Add file upload support for logo and favicon with automatic PNG conversion
- Add backend validation for brand asset data URLs (app_logo, app_favicon)
- Change vite_config.value column type from varchar(200) to text for PostgreSQL
- Add schema migration v3 for vite_config.value column type conversion
- Update frontend to use file picker instead of manual URL input
- Add early favicon application in index.html to prevent flash

go-backend/internal/store/repo/repository_migrate_test.go

@@ -83,3 +83,95 @@ func TestMigrateSchemaReturnsPostgresIDRepairError(t *testing.T) {
 		t.Fatalf("expected error %v, got %v", wantErr, err)
 	}
 }
+
+func TestMigrateSchemaRunsViteConfigValueMigrationForLegacySchema(t *testing.T) {
+	db, err := gorm.Open(gsqlite.Open(":memory:"), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	t.Cleanup(func() {
+		sqlDB, _ := db.DB()
+		if sqlDB != nil {
+			_ = sqlDB.Close()
+		}
+	})
+
+	if err := db.Exec(`CREATE TABLE schema_version (version INTEGER NOT NULL DEFAULT 0)`).Error; err != nil {
+		t.Fatalf("create schema_version: %v", err)
+	}
+	if err := db.Exec(`INSERT INTO schema_version(version) VALUES(?)`, 2).Error; err != nil {
+		t.Fatalf("seed schema_version: %v", err)
+	}
+
+	originalIDRepair := ensurePostgresIDDefaultsFn
+	ensurePostgresIDDefaultsFn = func(db *gorm.DB) error {
+		return nil
+	}
+	t.Cleanup(func() {
+		ensurePostgresIDDefaultsFn = originalIDRepair
+	})
+
+	called := 0
+	originalMigrate := migrateViteConfigValueColumnTypeFn
+	migrateViteConfigValueColumnTypeFn = func(db *gorm.DB) error {
+		called++
+		return nil
+	}
+	t.Cleanup(func() {
+		migrateViteConfigValueColumnTypeFn = originalMigrate
+	})
+
+	if err := migrateSchema(db); err != nil {
+		t.Fatalf("migrateSchema: %v", err)
+	}
+
+	if called != 1 {
+		t.Fatalf("expected vite_config migration to run once, got %d", called)
+	}
+}
+
+func TestMigrateSchemaReturnsViteConfigMigrationError(t *testing.T) {
+	db, err := gorm.Open(gsqlite.Open(":memory:"), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	t.Cleanup(func() {
+		sqlDB, _ := db.DB()
+		if sqlDB != nil {
+			_ = sqlDB.Close()
+		}
+	})
+
+	if err := db.Exec(`CREATE TABLE schema_version (version INTEGER NOT NULL DEFAULT 0)`).Error; err != nil {
+		t.Fatalf("create schema_version: %v", err)
+	}
+	if err := db.Exec(`INSERT INTO schema_version(version) VALUES(?)`, 2).Error; err != nil {
+		t.Fatalf("seed schema_version: %v", err)
+	}
+
+	originalIDRepair := ensurePostgresIDDefaultsFn
+	ensurePostgresIDDefaultsFn = func(db *gorm.DB) error {
+		return nil
+	}
+	t.Cleanup(func() {
+		ensurePostgresIDDefaultsFn = originalIDRepair
+	})
+
+	wantErr := errors.New("vite config migration failed")
+	originalMigrate := migrateViteConfigValueColumnTypeFn
+	migrateViteConfigValueColumnTypeFn = func(db *gorm.DB) error {
+		return wantErr
+	}
+	t.Cleanup(func() {
+		migrateViteConfigValueColumnTypeFn = originalMigrate
+	})
+
+	err = migrateSchema(db)
+	if !errors.Is(err, wantErr) {
+		t.Fatalf("expected error %v, got %v", wantErr, err)
+	}
+}