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/App.vue

@@ -79,6 +79,7 @@ function onGlobalKeydown(e: KeyboardEvent) {
       case "t": router.push("/tasks"); break;
       case "p": router.push("/projects"); break;
       case "c": router.push("/chat"); break;
+      case "g": router.push("/graph"); break;
     }
     return;
   }