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/extension/src/contribution.dart

@@ -27,6 +27,7 @@ class TabContribution extends ContributionPoint {
     required this.title,
     required this.build,
     this.icon,
+    this.iconColor,
     this.priority = 0,
     this.fileGlobs = const [],
     this.listenable,
@@ -39,6 +40,9 @@ class TabContribution extends ContributionPoint {
   final String title;
   final WidgetBuilder build;
   final Object? icon;
+
+  /// Optional identity tint for the icon-rail glyph (T-418).
+  final Color? iconColor;
   final int priority;
   final List<String> fileGlobs;
   final Listenable? listenable;