Self-service avatar picker in Account

Users can now set their own profile picture instead of asking staff:
POST /api/auth/avatar/ accepts a J-ID (or blank to clear) and reuses the
same item resolution as the staff endpoint. The Account page gains a
profile picture card with a searchable, paginated library grid — any
item works (the thumbnail is used), the current avatar is marked, and
the choice is confirmed before saving. Refreshing the signed-in user
updates the shell avatar immediately.

Verified against the dev server: set, clear, unknown J-ID -> 400,
anonymous -> 401.

ROADMAP.md

@@ -131,9 +131,10 @@ Files now stage first and are resolved before entering the library.
       (remembered per browser in `localStorage`)
 - [x] Toasts for action results (success/error) plus a shared confirm dialog for
       destructive actions; form-field validation stays inline
-- [ ] Mobile drawer polish for metadata panels (design spec §layout)
-- [x] Profile pictures: staff Users page sets avatars from library J-IDs
-      (self-service picker in Account still pending)
+- [x] Mobile bottom sheets for metadata panels (<768px): detail asides
+      (Library/Online/Similar) and long pool descriptions
+- [x] Profile pictures: staff Users page and a self-service Account picker
+      (searchable library grid, remove supported) set avatars from library J-IDs
 - [ ] Profile extras (per-user browse preferences)
 - [x] Command palette: tag finder, recent searches, navigation commands
       (Ctrl+K / ⌘K, reachable from anywhere in the shell)