feat: default tracker configs, email validation, expandable target links

- Tracker now has default_tracking_config_id and default_template_config_id
  that apply to all linked targets unless overridden per-target
- Dispatch falls back to tracker defaults when per-link configs are null
- Email bot creation validates SMTP connection before saving
- Email notifications sent as HTML (links render properly)
- Linked target items are expandable: collapsed shows config CrossLinks,
  expanded shows config selectors; action buttons always visible
- Fix email bot test button icon (mdiEmailSend → mdiSend)
- Fix target type icons in LinkedTargetsSection for all types
- Provider filter moved above search in sidebar

packages/server/src/notify_bridge_server/api/notification_trackers.py

@@ -32,6 +32,8 @@ class NotificationTrackerCreate(BaseModel):
     collection_ids: list[str] = []
     scan_interval: int = 60
     batch_duration: int = 0
+    default_tracking_config_id: int | None = None
+    default_template_config_id: int | None = None
     enabled: bool = True
 
 
@@ -41,6 +43,8 @@ class NotificationTrackerUpdate(BaseModel):
     collection_ids: list[str] | None = None
     scan_interval: int | None = None
     batch_duration: int | None = None
+    default_tracking_config_id: int | None = None
+    default_template_config_id: int | None = None
     enabled: bool | None = None
 
 
@@ -190,6 +194,8 @@ async def _tracker_response(session: AsyncSession, t: NotificationTracker) -> di
         "collection_ids": t.collection_ids,
         "scan_interval": t.scan_interval,
         "batch_duration": t.batch_duration,
+        "default_tracking_config_id": t.default_tracking_config_id,
+        "default_template_config_id": t.default_template_config_id,
         "enabled": t.enabled,
         "tracker_targets": tracker_targets,
         "created_at": t.created_at.isoformat(),