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/lib/api/downloads.ts

@@ -145,7 +145,7 @@ export interface ShareLink {
   // Null when the server cannot show this link's address again (no encrypted copy its key reads): the
   // link still opens for whoever has it; to share it anew, make a new link.
   token: string | null;
-  url: string | null; // path like "/watch/<token>" — prepend window.location.origin to share
+  url: string | null; // path like "/watch/<token>" or "/file/<token>" — prepend window.location.origin to share
   allow_download: boolean;
   has_password: boolean;
   expires_at: string | null;