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.

DorkNet.Server/site/src/components/Header.tsx

@@ -1,6 +1,7 @@
 import { NavLink, Link, useLocation, useNavigate } from 'react-router';
 import { useEffect, useRef, useState } from 'react';
 import { BrandMark } from './BrandMark';
+import { getDownloads } from '../lib/downloads';
 
 const NAV = [
   { to: '/', label: 'Home', exact: true },
@@ -10,6 +11,8 @@ const NAV = [
   { to: '/join', label: 'Join' },
   { to: '/quest', label: 'Quest' },
 ];
+// Shown only when the server offers downloads (DorkNet:Downloads:BaseUrl).
+const DOWNLOADS = { to: '/downloads', label: 'Downloads', exact: false };
 
 export function Header() {
   const [open, setOpen] = useState(false);
@@ -17,6 +20,12 @@ export function Header() {
   const nav = useNavigate();
   const loc = useLocation();
   const inputRef = useRef<HTMLInputElement>(null);
+  const [hasDownloads, setHasDownloads] = useState(false);
+  const navItems = hasDownloads ? [...NAV, DOWNLOADS] : NAV;
+
+  useEffect(() => {
+    getDownloads().then((m) => setHasDownloads(m !== null)).catch(() => {});
+  }, []);
 
   // Close the mobile drawer on route change so tapping a nav item
   // doesn't leave a half-open overlay covering the page.
@@ -37,7 +46,7 @@ export function Header() {
         </Link>
 
         <nav className="hidden md:flex items-center gap-1 ml-2">
-          {NAV.map(n => (
+          {navItems.map(n => (
             <NavLink
               key={n.to}
               to={n.to}
@@ -85,7 +94,7 @@ export function Header() {
               />
             </form>
             <nav className="flex flex-col gap-1">
-              {NAV.map(n => (
+              {navItems.map(n => (
                 <NavLink
                   key={n.to}
                   to={n.to}