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/lib/api/downloads.ts

@@ -118,6 +118,12 @@ export interface DownloadJob {
   position_seconds?: number;
 }
 
+// A row of the queue (the tray's poll): what is in flight or failed, and what finished a moment ago,
+// with where it came from as the library names it.
+export interface QueueJob extends DownloadJob {
+  source_key: string;
+}
+
 export interface ShareRecipient {
   id: number;
   email: string;
@@ -211,7 +217,7 @@ export const downloadsApi = {
     req("/api/downloads/clip", { method: "POST", body: JSON.stringify(body) }),
   downloadStoryboard: (id: number): Promise<StoryboardMeta> =>
     req(`/api/downloads/${id}/storyboard`),
-  downloads: (): Promise<DownloadJob[]> => req("/api/downloads"),
+  downloads: (): Promise<QueueJob[]> => req("/api/downloads"),
   downloadIndex: (): Promise<Record<string, DownloadStatus>> => req("/api/downloads/index"),
   pauseDownload: (id: number): Promise<DownloadJob> =>
     req(`/api/downloads/${id}/pause`, { method: "POST" }, { idempotent: true }),