Run the SPA without a backend (local mode)

The app can now operate backend-agnostically: a production build still
asks on first start, but /setup also offers 'Continue without a backend'
(stored as the sentinel 'none'), and the shell adapts:

- Local mode shows only the e621-facing pages: Online (search, post view,
  favorites, blacklist editor, direct downloads) and Pools. Library,
  uploads, duplicates, stats, users, follows and similarity are hidden
  from the nav and palette and render a 'backend needed' state when
  reached directly; /detail/<e621 id> still works while /detail/J-x asks
  for a backend.
- e621 credentials are stored in this browser (j621.e621) and the
  Account page becomes a credentials-only screen; the store reads/writes
  locally instead of /api/auth/e621/.
- The header replaces the status pill and login/user area with an e621
  credentials button and a 'Setup Backend' button; the footer shows
  'Local mode — e621 features only' with the same entry point.
- In-library lookups (badges/browse markers) are skipped without a
  backend; 'Download to client' links straight to the e621 file instead
  of the backend proxy; follow buttons and palette follow toggles are
  hidden; api() fails fast with a clear message if something slips
  through.

Mode logic lives in lib/backend.ts (URL / '' same-origin / 'none') with
its matrix verified in Node; tsc, oxlint and the build are clean.

ROADMAP.md

@@ -137,6 +137,9 @@ Files now stage first and are resolved before entering the library.
       (searchable library grid, remove supported) set avatars from library J-IDs
 - [x] Profile extras: per-user landing page, default rating filter/sort, items
       per page and thumbnail size (synced to the account, applied on load)
+- [x] Backend-less local mode: with no backend connected the SPA runs on the
+      e621-facing pages only (Online, Pools) using credentials stored in the
+      browser, and the shell offers a "Setup Backend" button instead
 - [x] Command palette: tag finder, recent searches, navigation commands
       (Ctrl+K / ⌘K, reachable from anywhere in the shell)