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/core/src/notify_bridge_core/notifications/dispatcher.py

@@ -27,7 +27,8 @@ class TargetConfig:
 
     type: str  # "telegram", "webhook", "email", "discord", "slack", "ntfy", "matrix"
     config: dict[str, Any]  # target-level config (bot_token, settings, etc.)
-    template_slots: dict[str, str] | None = None  # event_type -> template string
+    template_slots: dict[str, dict[str, str]] | None = None  # event_type -> {locale -> template}
+    locale: str = "en"  # preferred locale for template resolution
     date_format: str = "%d.%m.%Y, %H:%M UTC"
     date_only_format: str = "%d.%m.%Y"
     provider_api_key: str | None = None  # API key for downloading assets from provider
@@ -72,12 +73,12 @@ class NotificationDispatcher:
         self, event: ServiceEvent, target: TargetConfig
     ) -> dict[str, Any]:
         """Send event to a single target (potentially multiple receivers)."""
-        # Select template
+        # Select template with locale fallback
         template_str = DEFAULT_TEMPLATE
         if target.template_slots:
-            slot = target.template_slots.get(event.event_type.value)
-            if slot:
-                template_str = slot
+            locale_map = target.template_slots.get(event.event_type.value)
+            if locale_map:
+                template_str = locale_map.get(target.locale) or locale_map.get("en") or template_str
 
         # Build context and render ONCE
         ctx = build_template_context(