feat(site): Downloads page and Quest install from the latest build

When the server is pointed at the bucket CI publishes builds to
(DorkNet:Downloads:BaseUrl), the site lists them on a new Downloads page and
the Quest page offers "Latest build" next to "Choose file".

The server reads manifest.json and the files over HTTP on the internal network
and streams them through with Range support (DownloadsController); only files
the manifest lists are served. The xapk reader now reads from a byte source,
either the picked File or the server URL via Range requests, so the browser
installer streams the APK and OBB to the headset without downloading the
package first. Unset, both features stay hidden.

docs/quest.md

@@ -85,6 +85,13 @@ pitfall in their Quest client notes.
    installer scripts; the app appears as **DorkNet** under Unknown Sources.
    Share the `.xapk` however you like: the page reads it from the player's own
    disk, so your server never hosts game files.
+5. Optionally, let players install the latest CI build without downloading it
+   first: point the server at the downloads bucket CI publishes to
+   (`DorkNet__Downloads__BaseUrl`, see `deploy.md` → Client downloads). The
+   Quest page then offers **Latest build** next to **Choose file** and streams
+   the package from the server during the install, and the site gets a
+   **Downloads** page. The builds contain game files, so only do this with
+   the site behind an access proxy.
 
 No server component is Quest-specific beyond the placeholder-platform-id
-handling above.
+handling above and the optional downloads proxy.