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/DownloadDialog.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 Modal from "./Modal";
-import ProfileEditor from "./ProfileEditor";
 import { api } from "../lib/api";
+
+// Lazy: the full profile editor only opens from the dialog's "manage presets" affordance.
+const ProfileEditor = lazy(() => import("./ProfileEditor"));
 import { notify } from "../lib/notifications";
 import { navigateTo } from "../lib/nav";
 
@@ -105,12 +107,14 @@ export default function DownloadDialog({
       </div>
 
       {manage && (
-        <ProfileEditor
-          onClose={() => {
-            setManage(false);
-            profilesQ.refetch();
-          }}
-        />
+        <Suspense fallback={null}>
+          <ProfileEditor
+            onClose={() => {
+              setManage(false);
+              profilesQ.refetch();
+            }}
+          />
+        </Suspense>
       )}
     </Modal>
   );