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/package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "d3": "^7",
     "@tiptap/extension-link": "^2.11.0",
     "@tiptap/extension-placeholder": "^2.11.0",
     "@tiptap/pm": "^2.11.0",
@@ -22,6 +23,7 @@
     "vue-router": "^4.4.0"
   },
   "devDependencies": {
+    "@types/d3": "^7",
     "@types/dompurify": "^3.0.0",
     "@vitejs/plugin-vue": "^5.1.0",
     "typescript": "~5.6.0",