tint the Claude sidebar tab icon with the Claude accent (T-418)

TabContribution gains an optional iconColor honoured by the icon rail:
full-strength when active/hovered, dimmed (70%) when idle, so the tint
reads as identity without outshouting the active-state border. The
Claude Activity tab sets claudeAccent (#D97757) — nominative use per the
licenses.yaml trademark note (it marks Claude's own panel).

Filed and closed as a try-it-out (user request); trivially revertible if
the accent doesn't land visually.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/src/shell/layout.dart

@@ -146,7 +146,7 @@ class _BottomRail extends StatelessWidget {
         return Container(
           color: tokens.chromeBackground,
           child: ClideIconRail(
-            items: [for (final t in tabs) ClideIconRailItem(id: t.id, icon: _iconFor(slot, t), tooltip: resolveTabTitle(ctx, t))],
+            items: [for (final t in tabs) ClideIconRailItem(id: t.id, icon: _iconFor(slot, t), tooltip: resolveTabTitle(ctx, t), iconColor: t.iconColor)],
             activeId: activeId,
             onSelect: (id) => kernel.panels.activateTab(slot, id),
           ),