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/vite.config.ts

@@ -32,18 +32,18 @@ const proxy = {
   "/healthz": proxyOpts,
 };
 
-// Two HTML entries: the app (index.html) and a public /watch/<token> link (watch.html), which loads
+// Two HTML entries: the app (index.html) and a public share link (watch.html), which loads
 // only the player page. In a build the backend picks the file by path (`main.py`); this does the
 // same for `vite dev`, whose history fallback would otherwise hand /watch/… the app.
 const watchEntry = (): Plugin => ({
   name: "siftlode-watch-entry",
   configureServer(server) {
     server.middlewares.use((req, _res, next) => {
-      if (req.url?.startsWith("/watch/")) req.url = "/watch.html";
+      if (req.url && /^\/(?:watch|file|folder)\//.test(req.url)) req.url = "/watch.html";
       next();
     });
   },
-  // The server answers every /watch/ link with the built watch.html, so a build without it would
+  // The server answers every share link with the built watch.html, so a build without it would
   // ship a 500 for each one, and nothing downstream reads `dist` before prod does. Fail it here —
   // after Vite's own HTML plugin has emitted the pages (`enforce: "post"`), or none is there yet.
   enforce: "post",