feat(graph): register the vault graph view in the context panel (T-323)

Wires GraphPanel into GraphExtension as the graph.view context-panel tab
(its icon-rail entry and tab id were already scaffolded) and ships the
builtin.graph i18n catalogs. The graph is now reachable: notes as nodes,
wikilinks as edges, hover-highlight and click-to-open. It activates after
its pql dependency, which supplies the link data.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/graph/src/extension.dart

@@ -1,4 +1,7 @@
+import 'package:clide/builtin/graph/src/graph_panel.dart';
 import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
 
 class GraphExtension extends ClideExtension {
   @override
@@ -11,5 +14,16 @@ class GraphExtension extends ClideExtension {
   List<String> get dependsOn => const ['builtin.pql'];
 
   @override
-  List<ContributionPoint> get contributions => const [];
+  List<ContributionPoint> get contributions => [
+    TabContribution(
+      id: 'graph.view',
+      slot: Slots.contextPanel,
+      title: 'Graph',
+      titleKey: 'tab.graph.title',
+      i18nNamespace: id,
+      icon: PhosphorIcons.byName('graph'),
+      priority: -70,
+      build: (_) => const GraphPanel(),
+    ),
+  ];
 }