feat(ui): hub-endpoint settings dialog with persistence

Studio is no longer hardcoded to localhost:50051. A gear icon
in the sidebar footer opens a small settings dialog where the
operator picks host / port / TLS-on-or-off. Saved values
persist via shared_preferences and Studio reconnects to the
new endpoint immediately.

- data/hub.dart: HubService.loadPersistedEndpoint() called once
  before the first frame; reconnect() persists on every change.
- widgets/fai_settings_dialog.dart: typed form with port range
  validation, live URL preview pill, error surfacing if the
  reconnect fails.
- main.dart: gear icon at the sidebar bottom, replaces the
  static "platform v0.10.42" footer.

shared_preferences added as dependency. flutter analyze clean,
flutter test 2/2, macOS debug build succeeds.

Bumps fai_studio 0.4.0 -> 0.5.0.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/main.dart

@@ -16,7 +16,12 @@ import 'theme/theme.dart';
 import 'theme/tokens.dart';
 import 'widgets/widgets.dart';
 
-void main() {
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  // Restore the persisted endpoint before the first frame so the
+  // sidebar pill doesn't briefly say "127.0.0.1:50051" before
+  // switching to the operator's actual hub.
+  await HubService.instance.loadPersistedEndpoint();
   runApp(const StudioApp());
 }
 
@@ -199,15 +204,34 @@ class _Sidebar extends StatelessWidget {
               ],
             ),
           ),
-          // Footer hint.
+          // Footer: settings + version.
           Padding(
-            padding: const EdgeInsets.symmetric(horizontal: FaiSpace.lg),
-            child: Text(
-              'platform v0.10.42',
-              style: theme.textTheme.bodySmall?.copyWith(
-                color: theme.colorScheme.onSurfaceVariant,
-                fontSize: 10,
-              ),
+            padding: const EdgeInsets.symmetric(horizontal: FaiSpace.md),
+            child: Row(
+              children: [
+                Expanded(
+                  child: Text(
+                    'platform target',
+                    style: theme.textTheme.bodySmall?.copyWith(
+                      color: theme.colorScheme.onSurfaceVariant,
+                      fontSize: 10,
+                    ),
+                  ),
+                ),
+                IconButton(
+                  icon: const Icon(Icons.settings_outlined, size: 16),
+                  tooltip: 'Hub endpoint…',
+                  visualDensity: VisualDensity.compact,
+                  onPressed: () async {
+                    final saved = await FaiSettingsDialog.show(context);
+                    if (saved && context.mounted) {
+                      // Force a sidebar rebuild so the new endpoint
+                      // shows in the connection pill.
+                      (context as Element).markNeedsBuild();
+                    }
+                  },
+                ),
+              ],
             ),
           ),
         ],