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_card.dart

@@ -1,83 +0,0 @@
-// FaiCard — flat card with subtle accent gradient on the top
-// border. Replaces Material's default Card for the Ch∆In look.
-
-import 'package:flutter/material.dart';
-
-import '../theme/tokens.dart';
-
-class FaiCard extends StatelessWidget {
-  /// Card body. Padding is applied internally; pass plain content.
-  final Widget child;
-
-  /// When true, paint a 2px accent-coloured stripe along the top
-  /// edge. Used by foreground / "this matters" cards (a pending
-  /// approval, a failed step).
-  final bool accentTop;
-
-  /// When true, paint a faint vertical accent stripe on the left.
-  /// Used by status rows (live event, healthy service).
-  final Color? accentLeft;
-
-  /// Internal padding. Defaults to [FaiSpace.lg].
-  final EdgeInsetsGeometry padding;
-
-  const FaiCard({
-    super.key,
-    required this.child,
-    this.accentTop = false,
-    this.accentLeft,
-    this.padding = const EdgeInsets.all(FaiSpace.lg),
-  });
-
-  @override
-  Widget build(BuildContext context) {
-    final theme = Theme.of(context);
-    return Container(
-      decoration: BoxDecoration(
-        color: theme.colorScheme.surfaceContainer,
-        borderRadius: BorderRadius.circular(FaiRadius.md),
-        border: Border.all(color: theme.colorScheme.outlineVariant),
-      ),
-      clipBehavior: Clip.antiAlias,
-      child: Stack(
-        children: [
-          if (accentTop)
-            Positioned(
-              top: 0,
-              left: 0,
-              right: 0,
-              height: 2,
-              child: DecoratedBox(
-                decoration: BoxDecoration(
-                  gradient: LinearGradient(
-                    colors: [
-                      theme.colorScheme.primary.withValues(alpha: 0.0),
-                      theme.colorScheme.primary,
-                      theme.colorScheme.primary.withValues(alpha: 0.0),
-                    ],
-                  ),
-                ),
-              ),
-            ),
-          if (accentLeft != null)
-            Positioned(
-              top: FaiSpace.md,
-              bottom: FaiSpace.md,
-              left: 0,
-              width: 3,
-              child: DecoratedBox(
-                decoration: BoxDecoration(
-                  color: accentLeft,
-                  borderRadius: const BorderRadius.only(
-                    topRight: Radius.circular(2),
-                    bottomRight: Radius.circular(2),
-                  ),
-                ),
-              ),
-            ),
-          Padding(padding: padding, child: child),
-        ],
-      ),
-    );
-  }
-}