feat(files): API tokens in Settings, the Hub folder in Files, a notice before a token runs out

Settings gains an API tokens tab: a row per token (name and prefix; each
folder with its rights; last use and expiry, "expires soon" marked),
revoked from its menu behind a confirm. A new token is the Hub exchange in
one click, or folders and rights picked per folder (no Write where an
upload cannot land); expiry 30/90 days, 1 year or never (warned). The
token shows once in the same dialog, with Copy and a ready curl line.

The Hub folder closes the Sources in the Files rail, takes uploads and
moved-in files. A token about to run out warns its user once, a week
before (the Files GC's sixth pass); the notice opens the tokens tab.

frontend/src/design-lab/DesignLab.tsx

@@ -5,6 +5,7 @@ import { BatchB } from "./batchB";
 import { BatchC } from "./batchC";
 import { BatchD } from "./batchD";
 import { FilesRound } from "./filesRound";
+import { TokensRound } from "./tokensRound";
 import { Chip } from "../components/ui/Chip";
 import { Slider } from "../components/ui/NativeControls";
 import { SURFACE } from "../components/ui/surfaces";
@@ -75,6 +76,9 @@ export default function DesignLab() {
           />
         </label>
         <nav className="flex flex-wrap gap-1 text-xs">
+          <a href="#open-tokens" className="rounded-md px-1.5 py-0.5 text-accent state-layer">
+            Tokens
+          </a>
           <a href="#open-files" className="rounded-md px-1.5 py-0.5 text-accent state-layer">
             Files
           </a>
@@ -101,6 +105,14 @@ export default function DesignLab() {
             Resize the window to tablet size to see them there too.
           </p>
         </header>
+        <header id="open-tokens" className="mt-6 scroll-mt-20">
+          <h2 className="text-xl font-bold">Decided · API tokens, a machine&apos;s key to Files</h2>
+          <p className="text-sm text-muted">
+            Settings → API tokens. Picked: A, A, A — the variants with the accent edge (built; the
+            expiry is a segmented control, DESIGN.md "A choice between a few values").
+          </p>
+        </header>
+        <TokensRound />
         <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">