feat(files): the Downloads page goes — the tray, a Storage tab and a Downloads settings tab take its place

- The Downloads module, its rail icon and badge are gone. A bookmark, the remembered page and an old
  history entry that name it land on Files (urlState.resolvePage); `g d` goes to Files too.
- Users page: a Storage tab — the disk in numbers, every user's use against their own limit with
  their file count, every user's downloads; aligned tables, one menu per row (edit / reset quota,
  open source, copy, the owner's quota). The admin storage rows carry quota + file count, the admin
  job list its owner.
- Settings: a Downloads tab — the download formats in one list (duplicate a built-in, edit or delete
  your own) and the site sign-ins, moved from Account.
- "View" on an added download, a clip or a converting upload opens the tray on that row (unfolded,
  back if closed; a finished one opens as a file; under a player it opens without taking focus).
- The download dialog opens on the format used last.
- DataTable: `pager={false}` for a short list, `cardRow` to give a lone "…" its own card line.
- Welcome and README speak of Files; a new Files screenshot.

frontend/src/lib/nav.ts

@@ -1,21 +1,6 @@
 import type { Page } from "./urlState";
 
-// Tiny decoupled navigator so deep components (a feed card's download toast, the Download
-// Center) can switch pages without threading setPage through the whole tree. NavigationProvider
-// registers its setPage on mount; navigateTo is a no-op until then.
-let _navigate: ((p: Page) => void) | null = null;
-
-export function setNavigator(fn: (p: Page) => void): void {
-  _navigate = fn;
-}
-
-/** The "View" on a "Download added" toast is clicked INSIDE the player (its toast host). That is
- *  safe because `setPage` closes every open overlay before it pushes (see
- *  `lib/history.closeOverlaysThenNavigate`) — without it the player's Back-to-close cleanup popped the
- *  entry the navigation had just pushed. */
-export function navigateTo(p: Page): void {
-  _navigate?.(p);
-}
+// Page links: a page's own address, and whether a click on one is the app's to handle.
 
 /** A page's own address, for a link to it (the nav rail's rows). Only the page: the rest of a page's
  *  state — the feed's filters, Plex's library — a fresh tab restores from the account's own storage,