feat(files): download from anywhere — a link box, a pasted link, the palette; the running downloads in a tray

- Files toolbar link box (paste button, Enter); a lone http(s) link pasted onto any page opens the one
  app-wide download dialog, which now names the link; the palette's "Download from a link…" focuses
  the box on its history entry.
- The download tray, bottom right on every page: what waits, runs, is paused or failed (reason +
  Retry, the queue's right-click / "…" menu), and what finished in this tab as "Done · Open" until
  closed; folds to a pill (per-account pref); closed, it stays shut — across a reload — until the
  work changes or something finishes. Back to top steps up over it, the page end keeps clear.
- A finish is announced once: "Downloaded: …", whether a phone plays it, Copy link + Open (toasts
  and notices may carry a second action).
- The inbox's ready notice opens the file in its folder: download_done carries job_id + folder_id.
- GET /api/downloads is the queue only (in flight, failed, finished in the last 10 minutes), with
  each row's source key.

frontend/src/components/ui/pressableSelectable.contract.test.ts

@@ -17,6 +17,10 @@ const files = readSources(__dirname, [
 
 /** file → [how many plain Pressables, why they are not titles]. */
 const PLAIN: Record<string, [number, string]> = {
+  "DownloadTray.tsx": [
+    2,
+    "the folded pill (its summary is its label); a failure's reason, which opens its details",
+  ],
   "FileCard.tsx": [1, "a card's picture (named by aria-label; its title is the caption)"],
   "FilesRail.tsx": [2, "the load-error retry; the New-folder command"],
   "Home.tsx": [