v0.6.3: UI wording -> "messages"; folder spacing + tint

- User-facing "artifact(s)" -> "messages" across the UI (feed count, empty
  states, preview, About, Storage). Model/API/skill keep "artifact" internally
  — no agent-contract change.
- Sidebar: Keep/Deleted folders get spacing between them and share the "All
  sources" bold + blue-tint styling (special folders).
- Fix: About page points to Settings -> API Keys (was stale "Settings ->
  Account / API token").

152 tests pass; inline JS re-validated with node --check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/settings_demo_data.html

@@ -2,7 +2,7 @@
 
 {% block settings_content %}
   <h2>Demo Data</h2>
-  <p>Generate one sample artifact of every supported kind (markdown, HTML fragment, full HTML, deck, image, PDF, binary), tagged "demo" and dated today.</p>
+  <p>Generate one sample message of every supported kind (markdown, HTML fragment, full HTML, deck, image, PDF, binary), tagged "demo" and dated today.</p>
   <form method="post">
     {% csrf_token %}
     <button type="submit">Generate Demo Data</button>