fix(studio): chain config dir + spawn path + doc URLs

Track the platform rename: the hub spawn path is now ~/.chain/bin/chain
(was ~/.fai/bin/fai.exe on Windows — both dir and binary were stale, so
Studio could not launch the hub after the config-dir rename), the
~/.fai/* help strings become ~/.chain/*, FAI_REGISTRY_TOKEN ->
CHAIN_REGISTRY_TOKEN, and the two in-app doc URLs point at the public
fai/chain repo (fai/platform was renamed to the private fai/chain-private).
The .fai module bundle extension is left unchanged (format phase).
flutter analyze: no issues.

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

lib/data/hub_auth_token.dart

@@ -3,7 +3,7 @@ import 'package:path/path.dart' as p;
 
 /// Operator-managed gRPC bearer token used to authenticate
 /// Studio against a hub that has `auth.tokens:` configured
-/// (RBAC Level 2). Stored at `~/.fai/hub-auth-token` (mode
+/// (RBAC Level 2). Stored at `~/.chain/hub-auth-token` (mode
 /// 0600 on Unix). Sister of [RegistryToken] — same on-disk
 /// hygiene, different secret.
 ///
@@ -55,7 +55,7 @@ class HubAuthToken {
     return content.isEmpty ? null : content;
   }
 
-  /// Persist [token] to disk, creating `~/.fai/` if needed.
+  /// Persist [token] to disk, creating `~/.chain/` if needed.
   ///
   /// Atomic write: writes to `<path>.tmp`, chmods 0600 on
   /// Unix BEFORE moving into place, then renames. This
@@ -63,7 +63,7 @@ class HubAuthToken {
   /// `writeAsString` + later `chmod 600` sequence left the
   /// file world-readable for the duration of the chmod call.
   ///
-  /// Also sets `~/.fai/` itself to 0700 on Unix on first
+  /// Also sets `~/.chain/` itself to 0700 on Unix on first
   /// creation so other users on a shared host can't even
   /// enumerate the directory (the 0600 token-file mode is
   /// fine in isolation; combining it with a 0755 parent