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/BackToTop.tsx

@@ -4,7 +4,7 @@ import { ArrowUp } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import { Pressable } from "./Pressable";
 import { SURFACE } from "./surfaces";
-import { backToTopShown, useBackToTopShown } from "./backToTopState";
+import { backToTopShown, useBackToTopShown, useCornerLift } from "./backToTopState";
 
 // A floating "back to top" button that fades in once the current page's scroll container is
 // scrolled past a threshold, and smooth-scrolls it back to the top. Every page scrolls inside a
@@ -15,6 +15,8 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   const { t } = useTranslation();
   // Shared, not local: PageScroller keeps the page's end clear of the button while it is up.
   const show = useBackToTopShown();
+  // Over the download tray when it is up: the tray's 1rem inset, its height, then a gap.
+  const lift = useCornerLift();
   const scrollerRef = useRef<HTMLElement | null>(null);
 
   // Listen on the document in the CAPTURE phase and read the scroller off the event, rather than
@@ -63,6 +65,7 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
       // Hidden, it only goes transparent (so it can fade): out of the Tab order as well.
       tabIndex={show ? undefined : -1}
       aria-hidden={show ? undefined : true}
+      style={lift ? { bottom: `calc(${lift}px + 1.5rem)` } : undefined}
       className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center size-12 ${SURFACE.floating} glass-hover text-fg hover:text-accent transition-all duration-200 ${
         show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
       }`}