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/error_presentation.dart

@@ -6,7 +6,7 @@
 //   • a copyable, selectable rendition of the message
 //   • a friendly headline (gRPC codes mapped via
 //     friendlyError) instead of a wall-of-stack-trace
-//   • an automatic append to ~/.fai/logs/studio-errors.log so
+//   • an automatic append to ~/.chain/logs/studio-errors.log so
 //     the trail survives the dismiss action
 //
 // Call sites used to scatter `ScaffoldMessenger.of(ctx).showSnackBar(