feat(notifications): custom data fields for Home Assistant notify services (#1441)

When an HA notification provider targets a notify service (e.g.
notify.mobile_app_myphone), a new optional Data (JSON) field is
forwarded as the service call's nested "data" object - the same
place HA automations put mobile push options like priority, ttl,
channel, and group. ttl: 0 + priority: high make Android pushes
arrive immediately; channel gives printer alerts their own sound.

JSON rather than key=value lines so numbers stay numbers and nested
options work. Validated on both ends: the UI rejects malformed JSON
before saving, and the sender fails loudly instead of posting a
half-built payload. Only included when configured - the default
persistent_notification.create path is unchanged, as its schema
rejects unknown keys.

frontend/src/components/AddNotificationModal.tsx

@@ -144,6 +144,21 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
       }
     }
 
+    // HA custom service-data must be a JSON object (#1441)
+    if (providerType === 'homeassistant' && config.data?.trim()) {
+      let parsed: unknown;
+      try {
+        parsed = JSON.parse(config.data);
+      } catch {
+        setError(t('notifications.haDataInvalid'));
+        return;
+      }
+      if (typeof parsed !== 'object' || parsed === null || Array.isArray(parsed)) {
+        setError(t('notifications.haDataInvalid'));
+        return;
+      }
+    }
+
     const finalConfig: Record<string, unknown> =
       providerType === 'ntfy' && Object.keys(eventPriorities).length > 0
         ? { ...config, event_priorities: eventPriorities }
@@ -265,6 +280,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
       case 'homeassistant':
         return [
           { key: 'service', label: 'Home Assistant Service', placeholder: 'notify.mobile_app_myphone', type: 'text', required: false },
+          { key: 'data', label: 'Data (JSON, optional)', placeholder: '{"priority": "high", "ttl": 0, "channel": "3D Printing"}', type: 'textarea', required: false },
         ];
       default:
         return [];
@@ -368,6 +384,17 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
                       </option>
                     ))}
                   </select>
+                ) : field.type === 'textarea' ? (
+                  <textarea
+                    value={config[field.key] || ''}
+                    onChange={(e) => {
+                      setConfig({ ...config, [field.key]: e.target.value });
+                      setTestResult(null);
+                    }}
+                    placeholder={field.placeholder}
+                    rows={3}
+                    className="w-full px-3 py-2 bg-bambu-dark border border-bambu-dark-tertiary rounded-lg text-white focus:border-bambu-green focus:outline-none font-mono text-sm"
+                  />
                 ) : (
                   <input
                     type={field.type}