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/chain_empty_state.dart

@@ -0,0 +1,110 @@
+// ChainEmptyState — gracious empty / loading / error placeholder.
+// Replaces "(no data)" strings with one tone, one icon, one tip.
+//
+// The blank surface is where an app most easily feels unfinished,
+// so this state is deliberately *designed*: the Ch∆In delta (∆)
+// sits as a soft watermark behind the icon badge, the badge
+// carries a whisper of elevation, and the copy breathes. The
+// result reads "nothing here yet, on purpose" rather than
+// "something is broken".
+
+import 'package:flutter/material.dart';
+
+import '../theme/tokens.dart';
+import 'chain_delta_mark.dart';
+
+class ChainEmptyState extends StatelessWidget {
+  final IconData icon;
+  final String title;
+  final String? hint;
+  final Widget? action;
+
+  /// Icon tone. Defaults to muted; set to [Theme.colorScheme.error]
+  /// for connection-failure variants.
+  final Color? iconColor;
+
+  const ChainEmptyState({
+    super.key,
+    required this.icon,
+    required this.title,
+    this.hint,
+    this.action,
+    this.iconColor,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final color = iconColor ?? theme.colorScheme.onSurfaceVariant;
+    return Center(
+      child: ConstrainedBox(
+        constraints: const BoxConstraints(maxWidth: 440),
+        child: Padding(
+          padding: const EdgeInsets.all(ChainSpace.xxl),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              // Brand hero: an oversized, low-opacity ∆ watermark
+              // anchors the composition while the contextual icon
+              // badge sits crisply on top — so every empty surface
+              // still carries the Ch∆In signature, never a bare icon.
+              SizedBox(
+                width: 112,
+                height: 112,
+                child: Stack(
+                  alignment: Alignment.center,
+                  children: [
+                    Opacity(
+                      opacity: 0.10,
+                      child: ChainDeltaMark(
+                        color: theme.colorScheme.primary,
+                        size: 112,
+                      ),
+                    ),
+                    Container(
+                      width: 60,
+                      height: 60,
+                      decoration: BoxDecoration(
+                        color: theme.colorScheme.surfaceContainerHigh,
+                        shape: BoxShape.circle,
+                        border: Border.all(
+                          color: color.withValues(alpha: 0.18),
+                        ),
+                        boxShadow: ChainElevation.low(theme.brightness),
+                      ),
+                      child: Icon(icon, size: 26, color: color),
+                    ),
+                  ],
+                ),
+              ),
+              const SizedBox(height: ChainSpace.xl),
+              Text(
+                title,
+                textAlign: TextAlign.center,
+                style: theme.textTheme.titleMedium?.copyWith(
+                  fontWeight: FontWeight.w600,
+                ),
+              ),
+              if (hint != null) ...[
+                const SizedBox(height: ChainSpace.sm),
+                Text(
+                  hint!,
+                  textAlign: TextAlign.center,
+                  style: theme.textTheme.bodyMedium?.copyWith(
+                    color: theme.colorScheme.onSurfaceVariant,
+                    height: 1.5,
+                  ),
+                ),
+              ],
+              if (action != null) ...[
+                const SizedBox(height: ChainSpace.xl),
+                action!,
+              ],
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}