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/PageScroller.tsx

@@ -9,7 +9,7 @@ import {
 import { useScrollFade } from "../../lib/useScrollFade";
 import { FOCUS_REST_ATTRS, useFocusKeeper } from "../../lib/useFocusKeeper";
 import { StickyTopProvider } from "./stickyTop";
-import { useBackToTopShown } from "./backToTopState";
+import { useBackToTopShown, useCornerLift } from "./backToTopState";
 import { CONTENT_ID } from "./landmarks";
 import { ensureNavId, peekNavId, subscribeNav } from "../../lib/history";
 
@@ -139,6 +139,7 @@ export default function PageScroller({
   }, []);
   const { ref: fadeRef, style: fadeStyle } = useScrollFade(20, stickyTops === 0);
   const fabUp = useBackToTopShown();
+  const cornerLift = useCornerLift();
   const elRef = useRef<HTMLElement | null>(null);
   // Stable identity: an inline ref callback changes every render, which makes React detach and
   // re-attach the ref each commit — and since fadeRef is a state setter, that re-attach loops it
@@ -217,7 +218,15 @@ export default function PageScroller({
       {/* While Back to top floats over the corner, the page ends a button's height (3rem + its 1.25rem
           inset) later, so its last row or pager can scroll out from under it. Only then: a spacer
           always there would give a full-height layout a scrollbar it does not need. */}
-      {fabUp && <div aria-hidden="true" className="h-18" />}
+      {cornerLift > 0 ? (
+        // The download tray sits in the same corner (and the button, when up, over it).
+        <div
+          aria-hidden="true"
+          style={{ height: `calc(${cornerLift}px + ${fabUp ? "5.25rem" : "1.5rem"})` }}
+        />
+      ) : (
+        fabUp && <div aria-hidden="true" className="h-18" />
+      )}
     </main>
   );
 }