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/components/DownloadTray.tsx

@@ -195,7 +195,10 @@ export default function DownloadTray() {
         title,
       }),
       message: t(job.asset?.media_kind === "file" ? "tray.done.file" : "tray.done.phone"),
-      action: { label: t("tray.done.copyLink"), onClick: () => void quickShareLink(qc, job, t) },
+      action: {
+        label: t("tray.done.copyLink"),
+        onClick: () => void quickShareLink(qc, { kind: "file", id: job.id }, t),
+      },
       secondaryAction: {
         label: t("tray.done.open"),
         onClick: () => openFile(job.id, job.folder_id),
@@ -295,7 +298,7 @@ export default function DownloadTray() {
         id: "copyLink",
         label: t("tray.done.copyLink"),
         icon: <Link className="w-4 h-4" />,
-        onSelect: () => void quickShareLink(qc, job, t),
+        onSelect: () => void quickShareLink(qc, { kind: "file", id: job.id }, t),
       },
     ]);
   const menuItems = menu.target