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/data/fai_log.dart

@@ -1,12 +1,12 @@
 // Central error log for Studio. Every error surfaced to the
 // operator (SnackBar, error sheet, recoverable dialog) is also
-// appended here so `fai doctor` and "Open log" can show what
+// appended here so `chain doctor` and "Open log" can show what
 // went wrong without the operator having to reproduce the
 // failure.
 //
 // Path: `~/.fai/logs/studio-errors.log` (cross-platform via
 // HOME / USERPROFILE). One line per event, JSON-shaped so
-// `fai doctor` can parse it; humans can still read it because
+// `chain doctor` can parse it; humans can still read it because
 // the JSON is single-line and short.
 //
 // Best-effort by design: a failed write to the log file must
@@ -76,7 +76,7 @@ class FaiLog {
     return _append(entry);
   }
 
-  /// Read the last [maxLines] lines back. Used by `fai doctor`
+  /// Read the last [maxLines] lines back. Used by `chain doctor`
   /// + Settings → "Recent errors" view. Returns oldest first so
   /// the caller can render in chronological order.
   Future<List<String>> tail({int maxLines = 50}) async {