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/services/dispatch_helpers.py

@@ -129,18 +129,18 @@ async def load_link_data(
             tracking_config = await session.get(TrackingConfig, tt.tracking_config_id)
 
         template_config = None
-        template_slots: dict[str, str] | None = None
+        template_slots: dict[str, dict[str, str]] | None = None
         if tt.template_config_id:
             template_config = await session.get(TemplateConfig, tt.template_config_id)
             if template_config:
                 slot_result = await session.exec(
                     select(TemplateSlot).where(TemplateSlot.config_id == template_config.id)
                 )
-                raw_slots = {s.slot_name: s.template for s in slot_result.all()}
-                template_slots = {}
-                for slot_name, tmpl_text in raw_slots.items():
-                    event_key = slot_name.removeprefix("message_") if slot_name.startswith("message_") else slot_name
-                    template_slots[event_key] = tmpl_text
+                raw_slots: dict[str, dict[str, str]] = {}
+                for s in slot_result.all():
+                    event_key = s.slot_name.removeprefix("message_") if s.slot_name.startswith("message_") else s.slot_name
+                    raw_slots.setdefault(event_key, {})[s.locale] = s.template
+                template_slots = raw_slots
 
         target_config = dict(target.config)
         # Inject chat_action for Telegram targets