design(files): the one-library round in the lab — aligned list columns, the tray, the done notice

The Files page replaces the Downloads page: list rows with one column per value kind and a
right-click menu, an app-wide download tray, a done notice that copies the link. Picked
variants are logged in DESIGN.md.

frontend/src/design-lab/DesignLab.tsx

@@ -4,6 +4,7 @@ import { BatchA } from "./batchA";
 import { BatchB } from "./batchB";
 import { BatchC } from "./batchC";
 import { BatchD } from "./batchD";
+import { FilesRound } from "./filesRound";
 import { Chip } from "../components/ui/Chip";
 import { Slider } from "../components/ui/NativeControls";
 import { SURFACE } from "../components/ui/surfaces";
@@ -74,6 +75,9 @@ export default function DesignLab() {
           />
         </label>
         <nav className="flex flex-wrap gap-1 text-xs">
+          <a href="#open-files" className="rounded-md px-1.5 py-0.5 text-accent state-layer">
+            Files
+          </a>
           <a href="#decided" className="rounded-md px-1.5 py-0.5 text-success state-layer">
             A ✓
           </a>
@@ -97,6 +101,13 @@ export default function DesignLab() {
             Resize the window to tablet size to see them there too.
           </p>
         </header>
+        <header id="open-files" className="mt-6 scroll-mt-20">
+          <h2 className="text-xl font-bold">Open · Files, the one library</h2>
+          <p className="text-sm text-muted">
+            The page that replaces Downloads. The recommended variant has the accent edge.
+          </p>
+        </header>
+        <FilesRound />
         <header id="decided" className="mt-6 scroll-mt-20">
           <h2 className="text-xl font-bold">Decided · batch A, controls</h2>
           <p className="text-sm text-muted">