refactor: rename internal Fai* design system + fai_ helpers to chain

The Studio design system, widgets and helpers carried a Fai* / fai_
prefix (FaiSpace, FaiColors, FaiTheme, FaiLog, 17 fai_*.dart files, the
faiBinary* l10n keys). Studio is the Ch∆In product, so rename them to
Chain* / chain_ — carefully preserving English fail/failure/failed.
Also fix stale references: the 'fai' binary in l10n strings -> 'chain',
FAI_* env vars (FAI_BIN/DATA_DIR/MODULES_DIR/TODAY/BOOTSTRAP_TOKEN) ->
CHAIN_*, fai_platform -> fai_chain, fai_hub -> chain_hub. Vendor
security-hook tooling (FAI_BANNED_TERMS_FILE) + the .fai bundle ext left.
flutter analyze + test: clean (20 passed).

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

lib/data/theme_plugin.dart

@@ -16,7 +16,7 @@ import 'hub.dart';
 
 /// SharedPreferences key for the operator's preferred theme
 /// plugin. `null` (or absent) means "use Studio's built-in
-/// default" (FaiTheme.light / .dark).
+/// default" (ChainTheme.light / .dark).
 const _kThemePluginKey = 'theme_plugin_capability';
 
 /// One ColorScheme pair as built from a theme-plugin's
@@ -96,14 +96,14 @@ Future<ThemePluginSchemes> loadThemePluginSchemes(String capability) async {
 }
 
 /// Build a `ThemeData` from a plugin-supplied `ColorScheme`.
-/// Delegates to [FaiTheme.fromColorScheme] so plugin-themed
+/// Delegates to [ChainTheme.fromColorScheme] so plugin-themed
 /// builds and built-in themes share one structural shape — same
 /// textTheme cascade, same component themes, same `inherit`
 /// values on every TextStyle. Without that, Flutter's
 /// AnimatedDefaultTextStyle lerp blows up when swapping between
 /// the two paths.
 ThemeData themeDataFromScheme(ColorScheme scheme) =>
-    FaiTheme.fromColorScheme(scheme);
+    ChainTheme.fromColorScheme(scheme);
 
 /// Translate the plugin's 14 ARGB tokens into a `ColorScheme`.
 /// Order matches the plugin contract: