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.dart

@@ -55,7 +55,7 @@ class HubService {
   }
 
   /// Reconnect to a new endpoint and persist for next launch.
-  /// [authToken] is read from `~/.fai/hub-auth-token` by
+  /// [authToken] is read from `~/.chain/hub-auth-token` by
   /// default — pass `null` to drop a previously-loaded token,
   /// or omit the parameter to keep the current value.
   Future<void> reconnect(
@@ -241,7 +241,7 @@ class HubService {
   }
 
   /// Persist a new System-AI configuration. The hub writes back
-  /// to `~/.fai/config.yaml` and hot-reloads the in-memory copy
+  /// to `~/.chain/config.yaml` and hot-reloads the in-memory copy
   /// in one round-trip; the caller gets the resulting status
   /// for an immediate UI refresh.
   Future<SystemAiStatus> updateSystemAi({
@@ -449,7 +449,7 @@ class HubService {
 
   /// Read the installed module's `MODULE.md` (or
   /// `MODULE.<locale>.md`) from disk under
-  /// `~/.fai/modules/<module>/`. Air-gap friendly, no network.
+  /// `~/.chain/modules/<module>/`. Air-gap friendly, no network.
   ///
   /// `state` is one of:
   ///   - `'found'`     — `text` holds the markdown