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/pages/welcome.dart

@@ -1123,7 +1123,7 @@ class _DocReaderError {
   });
 
   String get forgejoUrl =>
-      'https://git.flemming.ai/fai/platform/src/branch/main/docs/architecture/$slug.md';
+      'https://git.flemming.ai/fai/chain/src/branch/main/docs/architecture/$slug.md';
 
   /// Localized, operator-facing failure body. Use this for any
   /// on-screen rendering; [toString] stays English for logs.