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/Files.menu.dom.test.tsx

@@ -151,6 +151,7 @@ const folder = (id: number, extra: Partial<FileFolder>): FileFolder => ({
   count: 1,
   size_bytes: 0,
   created_at: null,
+  has_link: false,
   ...extra,
 });
 const YT = folder(1, { source_key: "youtube" });
@@ -853,12 +854,14 @@ describe("Files · the grid", () => {
 describe("Files · the open folder's menu", () => {
   const folderMenu = () => screen.getByRole("button", { name: "files.folder.menu" });
 
-  it("an own folder is renamed, kept for a while, or deleted", async () => {
+  it("an own folder is shared by link, renamed, kept for a while, or deleted", async () => {
     mocks.files = [];
     mocks.folders = [YT, OWN];
     mount({ kind: "folder", id: 2 });
     await openMenu(() => fireEvent.click(folderMenu()));
     expect(menuRows()).toEqual([
+      "files.actions.copyLink",
+      "files.folder.shareLink",
       "files.folder.rename",
       "files.folder.retention",
       "files.folder.delete",