feat(files): share a folder by link — its page lists what is in it, plays a video above the list

A /watch link may now open one of its owner's own folders (migration 0108: folder_id beside job_id,
exactly one set; a folder's links go with it). The page shows what is in the folder when it is
opened, newest first: a video or a song plays in the same player a file link has, above the list,
each keeping its own resume point; a document only downloads, never opens as a page. A file moved
out stops opening through the link at its next request. Password, expiry and stream-only work as on
a file link; the unfurl names the folder and counts its files, never their names. Source folders
and the Hub cannot be shared.

The folder menu gets "Copy link" and "Share with a link…" (the Share dialog's link half); the rail
marks a folder a live link opens.

Two fixes in the Share dialog on the way: revoking the last link no longer mints a new one at once,
and the new-link form goes back to downloadable after making a link.

frontend/src/design-lab/DesignLab.tsx

@@ -6,6 +6,7 @@ import { BatchC } from "./batchC";
 import { BatchD } from "./batchD";
 import { FilesRound } from "./filesRound";
 import { TokensRound } from "./tokensRound";
+import { LinksRound } from "./linksRound";
 import { Chip } from "../components/ui/Chip";
 import { Slider } from "../components/ui/NativeControls";
 import { SURFACE } from "../components/ui/surfaces";
@@ -105,6 +106,14 @@ export default function DesignLab() {
             Resize the window to tablet size to see them there too.
           </p>
         </header>
+        <header id="open-links" className="mt-6 scroll-mt-20">
+          <h2 className="text-xl font-bold">Decided · Folder links</h2>
+          <p className="text-sm text-muted">
+            A /watch link that opens a whole folder. Picked: A, A, A — the variants with the accent
+            edge (built; on the public list a picture-less row keeps its glyph above the length).
+          </p>
+        </header>
+        <LinksRound />
         <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">