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/App.tsx

@@ -39,6 +39,8 @@ import DebugRecorder from "./components/DebugRecorder";
 import Toaster from "./components/ui/Toaster";
 import KeyboardHelp from "./components/ui/KeyboardHelp";
 import AppCommandPalette from "./components/AppCommandPalette";
+import DownloadIntake from "./components/DownloadIntake";
+import DownloadTray from "./components/DownloadTray";
 import { useGlobalKeymap } from "./lib/keymap";
 import ErrorDialog from "./components/ErrorDialog";
 import VersionBanner from "./components/VersionBanner";
@@ -461,12 +463,16 @@ export default function App() {
             Anchored inside the content column so they clear the sidebar automatically
             (collapsed or expanded) without tracking its width. */}
         <Toaster />
+        {/* The running downloads, bottom-right in the same column (the toasts keep the left). */}
+        <DownloadTray />
       </div>
       {/* The two app-wide overlays: the keyboard cheat sheet (`h`) and the jump-to palette (Mod+K).
           Mounted once, outside the page area, so every surface answers "what can I press here" and
           "take me to X" the same way. Their keys are bound by useGlobalKeymap above. */}
       <KeyboardHelp />
       <AppCommandPalette />
+      {/* The one download dialog: the Files link box, a link pasted onto any page, the palette. */}
+      <DownloadIntake />
       {/* The debug recorder: its start dialog, and its pill docked under the toasts. */}
       {me && <DebugRecorder />}
       <Suspense fallback={null}>