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/model/model.go

@@ -214,7 +214,7 @@ func (GroupPermissionGrant) TableName() string { return "group_permission_grant"
 type ViteConfig struct {
 	ID    int64  `gorm:"primaryKey;autoIncrement" json:"id"`
 	Name  string `gorm:"type:varchar(200);not null;uniqueIndex" json:"name"`
-	Value string `gorm:"type:varchar(200);not null" json:"value"`
+	Value string `gorm:"type:text;not null" json:"value"`
 	Time  int64  `gorm:"not null" json:"time"`
 }