fix(notifications): replace em dash in ntfy Title header (ASCII-only), improve error surfacing

HTTP headers are ASCII-only; the em dash in "PocketVeto — Test Notification"
caused a UnicodeEncodeError on every test attempt. Replaced with colon.

Frontend catch blocks now extract the real server error detail from the
axios response body instead of showing a generic fallback message.

Authored-By: Jack Levy

backend/app/api/notifications.py

@@ -116,7 +116,7 @@ async def test_ntfy(
         return NotificationTestResult(status="error", detail="Topic URL is required")
 
     headers: dict[str, str] = {
-        "Title": "PocketVeto — Test Notification",
+        "Title": "PocketVeto: Test Notification",
         "Priority": "default",
         "Tags": "white_check_mark",
     }