feat: add mobile responsiveness to settings layout

- Add mobile responsive styles for settings page
- Stack sidebar tabs horizontally on small screens
- Adjust workspace header for mobile
- Quality gates: typecheck ✓ lint ✓ build ✓

apps/web/src/styles.css

@@ -1136,6 +1136,41 @@ a {
   padding: 1.5rem;
 }
 
+/* Mobile responsiveness */
+@media (max-width: 768px) {
+  .settings-layout {
+    flex-direction: column;
+    gap: 1rem;
+  }
+
+  .settings-sidebar {
+    width: 100%;
+  }
+
+  .settings-nav {
+    flex-direction: row;
+    overflow-x: auto;
+    padding-bottom: 0.5rem;
+  }
+
+  .settings-nav-link {
+    white-space: nowrap;
+    padding: 0.5rem 0.75rem;
+    font-size: 0.875rem;
+  }
+
+  .workspace-header {
+    flex-direction: column;
+    gap: 1rem;
+    align-items: flex-start;
+  }
+
+  .workspace-header-actions {
+    width: 100%;
+    justify-content: flex-start;
+  }
+}
+
 .settings-panel h2 {
   margin: 0 0 1.25rem;
   font-size: 1.25rem;