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

frontend/app/notifications/page.tsx

@@ -89,8 +89,11 @@ export default function NotificationsPage() {
         ntfy_password: authMethod === "basic" ? password : "",
       });
       setNtfyTestResult(result);
-    } catch {
-      setNtfyTestResult({ status: "error", detail: "Request failed — check your topic URL" });
+    } catch (e: unknown) {
+      const detail =
+        (e as { response?: { data?: { detail?: string } } })?.response?.data?.detail
+        ?? (e instanceof Error ? e.message : "Request failed");
+      setNtfyTestResult({ status: "error", detail });
     } finally {
       setNtfyTesting(false);
     }
@@ -102,8 +105,11 @@ export default function NotificationsPage() {
     try {
       const result = await notificationsAPI.testRss();
       setRssTestResult(result);
-    } catch {
-      setRssTestResult({ status: "error", detail: "Feed check failed" });
+    } catch (e: unknown) {
+      const detail =
+        (e as { response?: { data?: { detail?: string } } })?.response?.data?.detail
+        ?? (e instanceof Error ? e.message : "Feed check failed");
+      setRssTestResult({ status: "error", detail });
     } finally {
       setRssTesting(false);
     }