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/pages/welcome.dart

@@ -1,5 +1,5 @@
 // Welcome — first surface a new operator sees, default sidebar
-// destination on launch. Pure introduction: what F∆I is, the
+// destination on launch. Pure introduction: what Ch∆In is, the
 // three concepts, the trust posture. No live data, no
 // federation chrome — that's the Store's role.
 //
@@ -1140,7 +1140,7 @@ class _DocReaderError {
       'Tried: ${attempted.join(", ")}\n'
       'Underlying: $underlying\n'
       'This typically means the Studio binary on disk is from\n'
-      'before the doc bundle changed. Rebuild Studio (fai studio\n'
+      'before the doc bundle changed. Rebuild Studio (chain studio\n'
       'or the build pipeline) or open the Forgejo copy in browser.';
 }