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/widgets/fai_log_viewer.dart

@@ -4,10 +4,10 @@
 // Use [showFaiLogViewer] to open it. Two log shapes are supported
 // out of the box:
 //
-//   - JSON-per-line (Studio's `~/.fai/logs/studio-errors.log`):
+//   - JSON-per-line (Studio's `~/.chain/logs/studio-errors.log`):
 //     fields `ts`, `level`, `source`, `error` get distinct colours;
 //     anything else renders neutral.
-//   - tracing text (the daemon's `~/.fai/run/<channel>.log`):
+//   - tracing text (the daemon's `~/.chain/run/<channel>.log`):
 //     `[INFO]`, `[WARN]`, `[ERROR]`, `[DEBUG]` substrings get the
 //     matching tone from the FaiTheme palette.
 //