M3 graph view: /api/graph + force-directed SVG

- graph blueprint: GET /api/graph resolves note_links to target notes by
  normalized title (owner-scoped, non-trashed, self-excluded) → {nodes, edges}
  of connected notes.
- GraphView: hand-rolled force simulation (repulsion + edge springs + centering,
  cooling over ~400 frames), SVG nodes/edges, click a node to open it (reuses the
  editor with link navigation). Sidebar Graph entry + route; empty state.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/router/index.ts

@@ -16,6 +16,7 @@ const router = createRouter({
         { path: "trash", name: "trash", component: () => import("../views/BoardView.vue") },
         { path: "label/:id", name: "label", component: () => import("../views/BoardView.vue") },
         { path: "search", name: "search", component: () => import("../views/SearchView.vue") },
+        { path: "graph", name: "graph", component: () => import("../views/GraphView.vue") },
       ],
     },
     {