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>

test/load_docs_test.dart

@@ -7,7 +7,7 @@ import 'package:chain_studio/l10n/app_localizations.dart';
 import 'package:chain_studio/theme/theme.dart';
 
 /// Recreates the exact DocReaderSheet code path that
-/// welcome.dart uses, including FaiTheme.markdownStyle, so a
+/// welcome.dart uses, including ChainTheme.markdownStyle, so a
 /// regression in either rootBundle, the markdown parser, or
 /// the style sheet surfaces here instead of only at runtime
 /// when an operator clicks a doc card.
@@ -16,7 +16,7 @@ void main() {
 
   testWidgets('DocReaderSheet renders all four german + english docs',
       (tester) async {
-    final theme = FaiTheme.light();
+    final theme = ChainTheme.light();
     for (final slug in ['architecture', 'security', 'audit', 'flows']) {
       for (final locale in ['de', 'en']) {
         final path = locale == 'de'
@@ -39,7 +39,7 @@ void main() {
               data: text,
               padding: const EdgeInsets.all(8),
               selectable: true,
-              styleSheet: FaiTheme.markdownStyle(theme),
+              styleSheet: ChainTheme.markdownStyle(theme),
             ),
           ),
         ));