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>

static/agentfeed.css

@@ -1139,20 +1139,25 @@ kbd {
   display: flex;
   flex-direction: column;
   flex-shrink: 0;
-  padding: 0.4rem 0.6rem 0.2rem;
+  gap: 0.25rem;   /* space Keep / Deleted apart */
+  padding: 0.4rem 0.6rem 0.3rem;
 }
 
-/* "All sources" — the primary entry, visually distinct from individual sources:
-   bold, with a subtle blue tint (works over both light and dark surfaces). */
-.all-sources-link {
+/* "Special" sidebar entries — "All sources" and the Keep/Deleted folders —
+   share a distinct look (bold + subtle blue tint) to set them apart from the
+   individual sources. */
+.all-sources-link,
+.folder-link {
   font-weight: 600;
 }
 
-.all-sources-link:not(.selected) {
+.all-sources-link:not(.selected),
+.folder-link:not(.selected) {
   background: rgba(74, 110, 168, 0.14);
 }
 
-.all-sources-link:not(.selected):hover {
+.all-sources-link:not(.selected):hover,
+.folder-link:not(.selected):hover {
   background: rgba(74, 110, 168, 0.22);
 }