feat: v2.8.0 — Artifacts uploads and plugin links, console tab combos, account avatars

- Artifacts hub: upload a file from the computer; make a skill by uploading a
  Markdown file, files, a folder or a .zip/.skill, or by writing it in the
  dialog; add plugins by picking from the list a marketplace link answers
  (POST /api/artifacts/plugin/inspect); Settings → AI tools → Artifacts Hub.
- Console tabs are combos of up to four split panes (ConsoleTabs, PaneGrid).
- Accounts: an own avatar (PUT/DELETE /api/me/avatar, raster only, ≤ 1 MB,
  <data dir>/avatars/), shown with name and role at the sidebar's foot and on
  the workspace owner groups (GET /api/users/:id/avatar, owner_avatar_at).
- Users page: per-user storage (GET /api/users/storage, measured in a worker)
  and account-pool spend/tokens per account (server/lib/pool-usage.js).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/db.js

@@ -1855,8 +1855,10 @@ try {
   }
 }
 // An account's own display name and email (the Users page's Edit user and
-// Settings → Account). Additive; after the rebuild above, which predates them.
-for (const col of ["display_name", "email"]) {
+// Settings → Account), and when its avatar was last set (server/lib/avatars.js
+// — the image is a file, the column only versions its URL). Additive; after
+// the rebuild above, which predates them.
+for (const col of ["display_name", "email", "avatar_at"]) {
   try {
     db.prepare(`SELECT ${col} FROM users LIMIT 1`).get();
   } catch {