feat(share): a link's address says what it opens

A video stays on /watch/, anything else is given out on /file/ and a folder on /folder/. The token
is the whole key, so every path opens every link and the links already sent keep working. The new
paths are kept out of search indexes like /watch/, boot the player alone, and the unfurl card calls a
file a file.

frontend/src/watch/WatchPage.tsx

@@ -28,7 +28,7 @@ import { SURFACE } from "../components/ui/surfaces";
 import { IconLink, LinkButton } from "../components/ui/LinkButton";
 import { T, lang } from "./watchStrings";
 
-// Standalone, login-free page for a shared /watch/<token> link: one file, or a folder's list of them.
+// Standalone, login-free page for a share link (/watch/, /file/, /folder/): one file, or a folder's list.
 // Its own entry (watchMain.tsx, watch.html): no auth, no providers, none of the app's bundle. Uses
 // plain fetch and a tiny self-contained i18n dict (`watchStrings`: the public viewer has no app
 // language preference).
@@ -83,8 +83,14 @@ async function notFoundState(r: Response): Promise<"preparing" | "gone"> {
   return body?.detail?.reason === "video_preparing" ? "preparing" : "gone";
 }
 
+/** The token of a share path: /watch/ for a video, /file/ or /folder/ for the rest, any of them for
+ * any link (`main.SHARE_PREFIXES`). */
+function shareToken(path: string): string {
+  return /^\/(?:watch|file|folder)\/([^/?#]+)/.exec(path)?.[1] ?? "";
+}
+
 export default function WatchPage() {
-  const token = window.location.pathname.split("/watch/")[1]?.split(/[/?#]/)[0] ?? "";
+  const token = shareToken(window.location.pathname);
   // Seeded from the URL rather than corrected by the effect below: with no token there is nothing
   // to load, and "loading" was committed and painted before the effect said so.
   const [status, setStatus] = useState<