perf: route- and modal-level code splitting (React.lazy)

The whole app shipped in one bundle, so the logged-out landing and every page
pulled all module code. Split into lazy chunks:
- main.tsx: lazy App + the public leaves (WatchPage, Privacy, Terms), so a
  public /watch share link never downloads the authenticated app.
- App.tsx: each module page (Feed, Channels, Playlists, Stats, Scheduler,
  Config, Users, Settings, Notifications, Messages, Downloads, ChannelPage) and
  the About/ReleaseNotes/Onboarding modals load on demand behind <Suspense>.
- Heavy modals lazy in their parents: PlayerModal (Feed, Playlists),
  DownloadDialog (DownloadButton — kept out of the feed chunk), and the
  VideoEditor/ShareDialog/ProfileEditor (DownloadCenter).
- Extracted focusAccessRequestsTab + the tab constants to lib/adminUsersTab so
  callers can pre-select the admin tab without statically importing the now
  lazy-loaded AdminUsers page (which would defeat the split).

Build now emits ~25 chunks. Landing no longer downloads any module code
(~350 KB deferred); dev landing Perf 93->95. Verified in a real browser: every
page + the video editor load with no console errors.

frontend/src/components/DownloadButton.tsx

@@ -1,9 +1,11 @@
-import { useState } from "react";
+import { lazy, Suspense, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Download } from "lucide-react";
 import clsx from "clsx";
-import DownloadDialog from "./DownloadDialog";
+// Lazy: the download dialog (+ its profile editor) loads only when a card's download button is
+// clicked, so it never ships in the feed chunk for the majority who just browse.
+const DownloadDialog = lazy(() => import("./DownloadDialog"));
 import { api, type Me } from "../lib/api";
 
 // Self-contained download affordance for a video card / the player. Hidden for the demo account
@@ -59,7 +61,9 @@ export default function DownloadButton({
         <Download className={clsx("w-4 h-4", inQueue && "animate-pulse")} />
       </button>
       {open && (
-        <DownloadDialog source={videoId} title={title} onClose={() => setOpen(false)} />
+        <Suspense fallback={null}>
+          <DownloadDialog source={videoId} title={title} onClose={() => setOpen(false)} />
+        </Suspense>
       )}
     </>
   );