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_storage.html

@@ -11,14 +11,14 @@
   </p>
 
   <ul class="storage-facts">
-    <li>{{ artifact_count }} artifact{{ artifact_count|pluralize }} total</li>
+    <li>{{ artifact_count }} message{{ artifact_count|pluralize }} total</li>
     <li>{{ kept_count }} kept ({{ kept_bytes|filesizeformat }}) &mdash; never auto-deleted</li>
   </ul>
 
   <p class="storage-note">
-    When your quota fills, the oldest <strong>unkept</strong> artifacts are deleted
-    automatically to make room for new ones. Click <strong>Keep</strong> on an artifact
-    to protect it. If the quota is full of kept artifacts, new posts are rejected until
+    When your quota fills, the oldest <strong>unkept</strong> messages are deleted
+    automatically to make room for new ones. Click <strong>Keep</strong> on a message
+    to protect it. If the quota is full of kept messages, new posts are rejected until
     you free space.
   </p>
 
@@ -26,7 +26,7 @@
     <h3>Admin</h3>
     <ul class="storage-facts">
       <li>Total quota allocated across all users: <strong>{{ total_allocated|filesizeformat }}</strong></li>
-      <li>Free space on the artifact disk: <strong>{{ disk_free|filesizeformat }}</strong></li>
+      <li>Free space on the storage disk: <strong>{{ disk_free|filesizeformat }}</strong></li>
     </ul>
     {% if total_allocated > disk_free %}
       <p class="storage-warning">