refactor(brand): rename F∆I -> Ch∆In + hub binary fai -> chain

Studio follows the platform rename: product branding F∆I -> Ch∆In in UI
strings, command examples fai -> chain, and — critically — the spawned
hub binary path ~/.fai/bin/fai -> ~/.fai/bin/chain so Studio launches
the renamed binary. The fai_* Dart identifiers (FaiLog, widget files,
the generated SDK) stay = vendor/internal namespace. flutter analyze:
no issues.

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

lib/main.dart

@@ -1,4 +1,4 @@
-// F∆I Studio — desktop GUI for the F∆I hub.
+// Ch∆In Studio — desktop GUI for the Ch∆In hub.
 //
 // Visual language: warm minimalism / deep tech with soul.
 // Tokens in lib/theme/, primitives in lib/widgets/.
@@ -215,7 +215,7 @@ class StudioAppState extends State<StudioApp> {
                 builder: (context, snap) {
                   final p = snap.data ?? (light: null, dark: null);
                   return MaterialApp(
-                    title: 'F∆I Studio',
+                    title: 'Ch∆In Studio',
                     debugShowCheckedModeBanner: false,
                     theme: p.light ?? FaiTheme.light(),
                     darkTheme: p.dark ?? FaiTheme.dark(),
@@ -1072,7 +1072,7 @@ class _BrandLabel extends StatelessWidget {
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
         const Text(
-          'F∆I Studio',
+          'Ch∆In Studio',
           style: TextStyle(fontSize: 16, fontWeight: FontWeight.w600),
           maxLines: 1,
           overflow: TextOverflow.ellipsis,