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/widgets/fai_en_badge.dart

@@ -1,59 +0,0 @@
-// FaiEnBadge — small "[EN]" pill shown next to text that came
-// from outside Studio's localization pipeline (MCP server tool
-// names, n8n endpoint descriptions, native LLM responses).
-//
-// The honest move: when the active locale isn't English and we
-// show a piece of text that we know is English, mark it so the
-// operator doesn't blame Studio for a half-translated page.
-// `studio.translate` plugin will eventually rewrite these in
-// place — until then, the badge is the right amount of
-// transparency.
-
-import 'package:flutter/material.dart';
-
-import '../l10n/app_localizations.dart';
-import '../theme/tokens.dart';
-
-class FaiEnBadge extends StatelessWidget {
-  /// When true, the badge renders. When false (e.g. the active
-  /// locale already is English), it returns
-  /// `SizedBox.shrink()` so callers can drop it inline without
-  /// guarding visibility themselves.
-  final bool visible;
-
-  const FaiEnBadge({super.key, required this.visible});
-
-  /// Convenience constructor: derives `visible` from the active
-  /// Localizations locale. Use this from inside a Build method
-  /// where you already have a BuildContext.
-  factory FaiEnBadge.forContext(BuildContext context) {
-    final lang = Localizations.localeOf(context).languageCode;
-    return FaiEnBadge(visible: lang != 'en');
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    if (!visible) return const SizedBox.shrink();
-    final theme = Theme.of(context);
-    final l = AppLocalizations.of(context)!;
-    return Tooltip(
-      message: l.mcpServerEnLanguageBadge,
-      child: Container(
-        padding: const EdgeInsets.symmetric(horizontal: 6, vertical: 1),
-        decoration: BoxDecoration(
-          color: theme.colorScheme.surfaceContainerHighest,
-          borderRadius: BorderRadius.circular(FaiRadius.sm),
-          border: Border.all(color: theme.colorScheme.outlineVariant),
-        ),
-        child: Text(
-          'EN',
-          style: theme.textTheme.labelSmall?.copyWith(
-            fontSize: 9,
-            letterSpacing: 0.4,
-            color: theme.colorScheme.onSurfaceVariant,
-          ),
-        ),
-      ),
-    );
-  }
-}