feat: note subtitle — show metadata (date + word count) (#94)

* feat: show metadata subtitle (date + word count) instead of snippet

Replace the note list subtitle with a compact metadata summary showing
relative date and word count (e.g. "2d ago · 342 words") instead of
the first paragraph of note content. Adds word_count to VaultEntry on
the Rust backend, computed during vault scan.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: cargo fmt formatting

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useVaultLoader.test.ts

@@ -10,7 +10,7 @@ const mockEntries: VaultEntry[] = [
     status: 'Active', owner: null, cadence: null,
     archived: false, trashed: false, trashedAt: null,
     modifiedAt: 1700000000, createdAt: 1700000000, fileSize: 100,
-    snippet: '', relationships: {}, icon: null, color: null, order: null, outgoingLinks: [],
+    snippet: '', wordCount: 0, relationships: {}, icon: null, color: null, order: null, outgoingLinks: [],
   },
 ]