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/Toaster.tsx

@@ -171,17 +171,25 @@ export default function Toaster({
                     </span>
                   )}
                 </div>
-                {toast.action && (
-                  <Button
-                    variant="link"
-                    onClick={() => {
-                      toast.action!.onClick();
-                      dismiss(toast.id);
-                    }}
-                    className="mt-1 text-sm"
-                  >
-                    {toast.action.label}
-                  </Button>
+                {(toast.action || toast.secondaryAction) && (
+                  <div className="mt-1 flex flex-wrap gap-x-4">
+                    {[toast.action, toast.secondaryAction].map(
+                      (action) =>
+                        action && (
+                          <Button
+                            key={action.label}
+                            variant="link"
+                            onClick={() => {
+                              action.onClick();
+                              dismiss(toast.id);
+                            }}
+                            className="text-sm"
+                          >
+                            {action.label}
+                          </Button>
+                        ),
+                    )}
+                  </div>
                 )}
               </div>
               <IconButton