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/integration/plugin_theme_test.dart

@@ -10,9 +10,9 @@
 //   flutter test test/integration/plugin_theme_test.dart
 //
 // Pre-reqs (handled by HubFixture's skip-when-missing path):
-//   - fai binary on PATH or at ../fai_platform/target/release/fai
+//   - fai binary on PATH or at ../chain_platform/target/release/fai
 //   - studio-theme-solarized plugin built to module.wasm in its
-//     fai_plugins dir.
+//     chain_plugins dir.
 
 import 'dart:io';
 
@@ -33,7 +33,7 @@ void main() {
         // add network + bundle-fetch latency that doesn't
         // belong in an in-process round-trip test.
         final pluginSrc = Directory(
-          '../fai_plugins/studio-theme-solarized',
+          '../chain_plugins/studio-theme-solarized',
         );
         if (!await pluginSrc.exists()) {
           markTestSkipped('studio-theme-solarized source not available');