Add test notification endpoint and UI button

- Add POST /api/test-notification endpoint to send test to saved config URL
- Refactor send_notification to return Result instead of bool
- Add NotificationError enum for proper error handling
- Add test notification button in config UI with explanatory text
- Button tests saved configuration URL, not input field value

daemon/web/src/lib/utils.svelte.ts

@@ -86,3 +86,14 @@ export async function set_config(config: Config): Promise<void> {
         throw new Error(error);
     }
 }
+
+export async function test_notification(): Promise<void> {
+    const response = await fetch('/api/test-notification', {
+        method: 'POST',
+    });
+
+    if (!response.ok) {
+        const error = await response.text();
+        throw new Error(error);
+    }
+}