fix(graph): default /api/graph to newest-memory center, add sort param

memory_timeline PR #37 exposed the same class of bug in the graph
endpoint: the dashboard Graph page (and the /api/graph endpoint it
hits) defaulted to centering on the most-connected memory, ran BFS at
depth 3, and capped the subgraph at 150 nodes. On a mature corpus this
clustered the visualization around a historical hotspot and hid freshly
ingested memories that hadn't accumulated edges yet. User-visible
symptom: TimeSlider on /graph showing "Feb 21 → Mar 1 2026" when the
database actually contains memories through today (Apr 20).

**Backend (`crates/vestige-mcp/src/dashboard/handlers.rs`):**
- `GraphParams` gains `sort: Option<String>` (accepted: "recent" |
  "connected", unknown falls back to "recent").
- New internal `GraphSort` enum + case-insensitive `parse()`.
- Extracted `default_center_id(storage, sort)` so handler logic and
  tests share the same branching. Recent path picks `get_all_nodes(1,
  0)` (ORDER BY created_at DESC). Connected path picks
  `get_most_connected_memory`, degrading gracefully to recent if the
  DB has no edges yet.
- Default behaviour flipped from "connected" to "recent" — matches
  user expectation of "show me my recent stuff".

**Dashboard (`apps/dashboard`):**
- `api.graph()` accepts `sort?: 'recent' | 'connected'` with JSDoc
  explaining the rationale.
- `/graph/+page.svelte` passes `sort: 'recent'` when no query or
  center_id is active. Query / center_id paths unchanged — they
  already carry their own centering intent.

**Tests:** 6 new unit tests in `handlers::tests`:
- `graph_sort_parse_defaults_to_recent` (None, empty, garbage,
  "recent", "Recent", "RECENT")
- `graph_sort_parse_accepts_connected_case_insensitive`
- `default_center_id_recent_returns_newest_node` — ingest 3 nodes,
  assert newest is picked
- `default_center_id_connected_prefers_hub_over_newest` — wire a hub
  node with 3 spokes, then ingest a newer "lonely" node; assert the
  hub wins in Connected mode even though it's older
- `default_center_id_connected_falls_back_to_recent_when_no_edges`
  — fresh DB with no connections still returns newest, not 404
- `default_center_id_returns_not_found_on_empty_db` — both modes
  return 404 cleanly on empty storage

Build + test:
- cargo test -p vestige-mcp --lib handlers:: → 6/6 pass
- cargo test --workspace --lib → 830/830 pass, 0 failed
- cargo clippy -p vestige-core -p vestige-mcp --lib -- -D warnings →
  clean
- npm run check → 0 errors, 0 warnings
- npm test → 361/361 pass

Binary already installed at ~/.local/bin/vestige-mcp (copied from
cargo build --release -p vestige-mcp). New Claude Desktop / Code
sessions will pick it up automatically when they respawn their MCP
subprocess. The dashboard HTTP server on port 3927 needs a manual
relaunch from a terminal with the usual pattern:

    nohup bash -c 'tail -f /dev/null | \
        VESTIGE_DASHBOARD_ENABLED=true ~/.local/bin/vestige-mcp' \
        > /tmp/vestige-mcp.log 2>&1 & disown

apps/dashboard/src/lib/stores/api.ts

@@ -63,7 +63,21 @@ export const api = {
 		fetcher<TimelineResponse>(`/timeline?days=${days}&limit=${limit}`),
 
 	// Graph
-	graph: (params?: { query?: string; center_id?: string; depth?: number; max_nodes?: number }) => {
+	//
+	// `sort` controls the default center when no query/center_id is given:
+	//   - "recent" (default) — newest memory; matches user expectation of
+	//     "show me what I just added". Previously the backend defaulted to
+	//     "connected" which clustered on historical hotspots and hid
+	//     fresh memories that hadn't accumulated edges yet.
+	//   - "connected" — densest node; richer initial subgraph for a
+	//     well-aged corpus. Exposed for a future UI toggle.
+	graph: (params?: {
+		query?: string;
+		center_id?: string;
+		depth?: number;
+		max_nodes?: number;
+		sort?: 'recent' | 'connected';
+	}) => {
 		const qs = params ? '?' + new URLSearchParams(
 			Object.entries(params)
 				.filter(([, v]) => v !== undefined)