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.

backend/app/routes/files.py

@@ -556,7 +556,7 @@ def list_files(
     assets = assets_for(db, jobs)
     positions = positions_for(db, user, jobs)
     shared_at = _shared_at(db, user, jobs) if shared else {}
-    sources = _source_keys(db, jobs)
+    sources = source_keys(db, jobs)
     return [
         {
             **serialize(j, assets.get(j.asset_id) if j.asset_id is not None else None),
@@ -569,9 +569,9 @@ def list_files(
     ]
 
 
-def _source_keys(db: Session, jobs: Sequence[DownloadJob]) -> dict[int, str]:
-    """Where each file came from (its source folder's key), for the list's Source column and the
-    "back to its source folder" move: a file in a source folder has its folder's key; one moved into
+def source_keys(db: Session, jobs: Sequence[DownloadJob]) -> dict[int, str]:
+    """Where each file came from (its source folder's key), for the list's Source column, the
+    "back to its source folder" move and the download tray's site: a file in a source folder has its folder's key; one moved into
     an own folder (or someone else's, shared) is traced back by `folders.source_key_for`."""
     ids = {j.folder_id for j in jobs if j.folder_id is not None}
     by_id = (