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/components/Editor.test.tsx

@@ -70,6 +70,7 @@ const mockEntry: VaultEntry = {
   createdAt: null,
   fileSize: 1024,
   snippet: '',
+  wordCount: 0,
   relationships: {},
   icon: null,
   color: null,