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/components/AppShell.vue

@@ -97,6 +97,9 @@ async function signOut() {
           <RouterLink to="/" class="nav-link" :class="route.name === 'board' ? 'nav-link-active' : ''">
             <Icon name="note" /> Notes
           </RouterLink>
+          <RouterLink to="/graph" class="nav-link" :class="route.name === 'graph' ? 'nav-link-active' : ''">
+            <Icon name="graph" /> Graph
+          </RouterLink>
 
           <div class="mt-3 flex items-center justify-between px-3 pb-1">
             <span class="text-xs font-semibold uppercase tracking-wide text-neutral-400">Labels</span>