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_pill.dart

@@ -1,75 +0,0 @@
-// FaiPill — small inline label. Used for capabilities, versions,
-// permission scopes, status text. Replaces ad-hoc Container chips.
-
-import 'package:flutter/material.dart';
-
-import '../theme/theme.dart';
-import '../theme/tokens.dart';
-
-enum FaiPillTone { neutral, accent, success, warning, danger }
-
-class FaiPill extends StatelessWidget {
-  final String label;
-  final FaiPillTone tone;
-  final IconData? icon;
-  final bool monospace;
-
-  const FaiPill({
-    super.key,
-    required this.label,
-    this.tone = FaiPillTone.neutral,
-    this.icon,
-    this.monospace = false,
-  });
-
-  @override
-  Widget build(BuildContext context) {
-    final theme = Theme.of(context);
-    final (bg, fg) = _colors(theme.colorScheme);
-    final textStyle = monospace
-        ? FaiTheme.mono(size: 11, weight: FontWeight.w500, color: fg)
-        : theme.textTheme.labelSmall?.copyWith(color: fg);
-    return Container(
-      padding: const EdgeInsets.symmetric(horizontal: FaiSpace.sm, vertical: 3),
-      decoration: BoxDecoration(
-        color: bg,
-        borderRadius: BorderRadius.circular(FaiRadius.sm),
-      ),
-      child: Row(
-        mainAxisSize: MainAxisSize.min,
-        children: [
-          if (icon != null) ...[
-            Icon(icon, size: 11, color: fg),
-            const SizedBox(width: 4),
-          ],
-          Text(label, style: textStyle),
-        ],
-      ),
-    );
-  }
-
-  (Color, Color) _colors(ColorScheme scheme) {
-    switch (tone) {
-      case FaiPillTone.neutral:
-        return (scheme.surfaceContainerHighest, scheme.onSurfaceVariant);
-      case FaiPillTone.accent:
-        return (scheme.primary.withValues(alpha: 0.15), scheme.primary);
-      case FaiPillTone.success:
-        return (
-          FaiColors.success.withValues(alpha: 0.15),
-          scheme.brightness == Brightness.dark
-              ? FaiColors.success
-              : const Color(0xFF15803D),
-        );
-      case FaiPillTone.warning:
-        return (
-          FaiColors.warning.withValues(alpha: 0.15),
-          scheme.brightness == Brightness.dark
-              ? FaiColors.warning
-              : const Color(0xFFB45309),
-        );
-      case FaiPillTone.danger:
-        return (scheme.errorContainer, scheme.onErrorContainer);
-    }
-  }
-}