Fix permissions handling and surface write errors to the UI

- Server attempts chmod on data dir at startup if not writable
- All write routes now catch errors and return 500 with a readable message
- Added /api/health endpoint to diagnose mount issues
- Frontend propagates server errors to a visible banner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DReRqHxSYuVfCzTzQGD9hZ

src/App.css

@@ -234,6 +234,31 @@ body {
   gap: 0.75rem;
 }
 
+.error-banner {
+  background: #fef2f2;
+  border: 1px solid #fca5a5;
+  color: #b91c1c;
+  border-radius: 8px;
+  padding: .75rem 1rem;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  font-size: .88rem;
+  margin-bottom: 1rem;
+}
+
+@media (prefers-color-scheme: dark) {
+  .error-banner {
+    background: #2d1515;
+    border-color: #7f1d1d;
+    color: #fca5a5;
+  }
+}
+
+:root[data-theme="dark"] .error-banner {
+  background: #2d1515;
+  border-color: #7f1d1d;
+  color: #fca5a5;
+}
+
 .loading {
   text-align: center;
   color: var(--text-muted);