feat: locale-aware notification templates + UX improvements

- Add locale support to notification templates (matching command template
  pattern): TemplateSlot now has locale field with (config_id, slot_name,
  locale) uniqueness, nested API format {slot: {locale: template}}
- Migration merges separate EN/RU system configs into unified per-provider
  configs; seeds create one config per provider with multi-locale slots
- Locale-aware dispatch with EN fallback in NotificationDispatcher
- Frontend locale tabs (EN/RU) on template config editor
- Fix tracking config cards not showing default provider icons
- Global provider filter, search palette, and various UX polish

packages/server/src/notify_bridge_server/main.py

@@ -47,7 +47,7 @@ async def lifespan(app: FastAPI):
     await init_db()
     # Run data migrations (idempotent)
     from .database.engine import get_engine
-    from .database.migrations import migrate_schema, migrate_tracker_targets, migrate_entity_refactor, migrate_template_slots, migrate_target_receivers, migrate_template_locale, migrate_receivers_from_config, migrate_command_slot_locale
+    from .database.migrations import migrate_schema, migrate_tracker_targets, migrate_entity_refactor, migrate_template_slots, migrate_target_receivers, migrate_template_locale, migrate_receivers_from_config, migrate_command_slot_locale, migrate_notification_slot_locale
     engine = get_engine()
     await migrate_schema(engine)
     await migrate_tracker_targets(engine)
@@ -57,6 +57,7 @@ async def lifespan(app: FastAPI):
     await migrate_template_locale(engine)
     await migrate_receivers_from_config(engine)
     await migrate_command_slot_locale(engine)
+    await migrate_notification_slot_locale(engine)
     from .database.seeds import seed_all
     await seed_all()
     # Configure webhook secret from DB setting (falls back to env var)