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

@@ -274,6 +274,7 @@ class NotificationDispatcher:
                 to_email=receiver.email,
                 subject=subject,
                 body_text=message,
+                body_html=message,
                 to_name=receiver.name,
             )
             results.append(result)

packages/core/src/notify_bridge_core/notifications/email/client.py

@@ -30,6 +30,33 @@ class EmailClient:
     def __init__(self, smtp_config: SmtpConfig) -> None:
         self._config = smtp_config
 
+    async def verify_connection(self) -> dict[str, Any]:
+        """Test SMTP connection and authentication without sending an email."""
+        try:
+            import aiosmtplib
+        except ImportError:
+            return {"success": False, "error": "aiosmtplib not installed"}
+
+        cfg = self._config
+        if not cfg.host:
+            return {"success": False, "error": "SMTP host not configured"}
+
+        try:
+            smtp = aiosmtplib.SMTP(
+                hostname=cfg.host,
+                port=cfg.port,
+                use_tls=cfg.use_tls,
+                start_tls=not cfg.use_tls and cfg.port != 25,
+            )
+            await smtp.connect()
+            if cfg.username and cfg.password:
+                await smtp.login(cfg.username, cfg.password)
+            await smtp.quit()
+            return {"success": True}
+        except Exception as e:
+            _LOGGER.warning("SMTP verification failed for %s:%d: %s", cfg.host, cfg.port, e)
+            return {"success": False, "error": str(e)}
+
     async def send(
         self,
         to_email: str,