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/components/AppHeader.vue

@@ -88,6 +88,7 @@ onUnmounted(() => document.removeEventListener("click", handleClickOutside));
         <router-link to="/projects" class="nav-link">Projects</router-link>
         <router-link to="/tasks" class="nav-link">Tasks</router-link>
         <router-link to="/chat" class="nav-link">Chat</router-link>
+        <router-link to="/graph" class="nav-link">Graph</router-link>
       </div>
 
       <!-- Right: status + utilities + gear + user -->
@@ -139,6 +140,7 @@ onUnmounted(() => document.removeEventListener("click", handleClickOutside));
       <router-link to="/projects" class="nav-link">Projects</router-link>
       <router-link to="/tasks" class="nav-link">Tasks</router-link>
       <router-link to="/chat" class="nav-link">Chat</router-link>
+      <router-link to="/graph" class="nav-link">Graph</router-link>
       <div class="mobile-divider"></div>
       <router-link to="/settings" class="nav-link">Settings</router-link>
       <router-link v-if="authStore.isAdmin" to="/admin/users" class="nav-link">Users</router-link>