Force-directed graph view with tag nodes, project clustering, physics tuning

- New /graph route with D3 force simulation (GraphView.vue)
- Tag nodes as first-class graph nodes (string IDs "tag:name") — clicking
  navigates to /notes?tags=name; tags shown by default
- Invisible project hub nodes attract project members into clusters
- Physics panel with live sliders: repulsion, link distance, link strength,
  project pull, gravity (forceX/forceY, not forceCenter)
- Wikilink edges retain directed arrowheads; tag edges are thin, no arrowhead
- Graph nav link in AppHeader; `g` shortcut in App.vue
- Backend: build_note_graph() emits tag nodes + note→tag edges instead of
  O(n²) note→note shared-tag mesh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -60,6 +60,11 @@ const router = createRouter({
       name: "note-edit",
       component: () => import("@/views/NoteEditorView.vue"),
     },
+    {
+      path: "/graph",
+      name: "graph",
+      component: () => import("@/views/GraphView.vue"),
+    },
     {
       path: "/projects",
       name: "projects",