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/widgets/fai_empty_state.dart

@@ -2,7 +2,7 @@
 // Replaces "(no data)" strings with one tone, one icon, one tip.
 //
 // The blank surface is where an app most easily feels unfinished,
-// so this state is deliberately *designed*: the F∆I delta (∆)
+// so this state is deliberately *designed*: the Ch∆In delta (∆)
 // sits as a soft watermark behind the icon badge, the badge
 // carries a whisper of elevation, and the copy breathes. The
 // result reads "nothing here yet, on purpose" rather than
@@ -48,7 +48,7 @@ class FaiEmptyState extends StatelessWidget {
               // Brand hero: an oversized, low-opacity ∆ watermark
               // anchors the composition while the contextual icon
               // badge sits crisply on top — so every empty surface
-              // still carries the F∆I signature, never a bare icon.
+              // still carries the Ch∆In signature, never a bare icon.
               SizedBox(
                 width: 112,
                 height: 112,