feat(phase3): import/export, sparklines, user theme overrides

- JSON import/export with conflict resolution (skip/overwrite) + admin UI
- Ping history sparklines on AppWidget and AppCard (24h, 288 points)
- Hourly cleanup job for old AppStatus records
- User theme preferences (hue, saturation, mode, background, locale)
- Settings page with ThemeCustomizer (sliders, toggles, live preview)
- Prisma migration for user preference fields
- i18n translations for all new strings (EN/RU)

prisma/migrations/20260324212043_add_user_preferences/migration.sql

@@ -0,0 +1,6 @@
+-- AlterTable
+ALTER TABLE "User" ADD COLUMN "backgroundType" TEXT;
+ALTER TABLE "User" ADD COLUMN "locale" TEXT;
+ALTER TABLE "User" ADD COLUMN "primaryHue" INTEGER;
+ALTER TABLE "User" ADD COLUMN "primarySaturation" INTEGER;
+ALTER TABLE "User" ADD COLUMN "themeMode" TEXT;

prisma/schema.prisma

@@ -20,6 +20,12 @@ model User {
   createdAt             DateTime @default(now())
   updatedAt             DateTime @updatedAt
 
+  themeMode         String?
+  primaryHue        Int?
+  primarySaturation Int?
+  backgroundType    String?
+  locale            String?
+
   groups      UserGroup[]
   createdApps App[]
   boards      Board[]