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/email_bots.py

@@ -55,6 +55,18 @@ async def create_email_bot(
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
 ):
+    # Verify SMTP connection before saving
+    from notify_bridge_core.notifications.email.client import EmailClient, SmtpConfig
+    client = EmailClient(SmtpConfig(
+        host=body.smtp_host, port=body.smtp_port,
+        username=body.smtp_username, password=body.smtp_password,
+        from_address=body.email, from_name=body.name,
+        use_tls=body.smtp_use_tls,
+    ))
+    result = await client.verify_connection()
+    if not result.get("success"):
+        raise HTTPException(status_code=400, detail=f"SMTP connection failed: {result.get('error', 'Unknown error')}")
+
     bot = EmailBot(user_id=user.id, **body.model_dump())
     session.add(bot)
     await session.commit()