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/Feed.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useRef, useState } from "react";
+import { lazy, Suspense, useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { keepPreviousData, useInfiniteQuery, useQuery, useQueryClient } from "@tanstack/react-query";
 import { ArrowDown, ArrowUp, ArrowLeft, Info, RefreshCw, Trash2, Youtube } from "lucide-react";
@@ -7,7 +7,8 @@ import i18n from "../i18n";
 import { notify, resolveVideo } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import VirtualFeed from "./VirtualFeed";
-import PlayerModal from "./PlayerModal";
+// Lazy: the in-app YouTube player (IFrame API) loads only when a video is first opened.
+const PlayerModal = lazy(() => import("./PlayerModal"));
 
 const PAGE = 60;
 
@@ -441,13 +442,15 @@ export default function Feed({
         )}
 
         {activeVideo && (
-          <PlayerModal
-            video={activeVideo.video}
-            startAt={activeVideo.startAt}
-            onClose={() => setActiveVideo(null)}
-            onState={onState}
-            onOpenChannel={onOpenChannel}
-          />
+          <Suspense fallback={null}>
+            <PlayerModal
+              video={activeVideo.video}
+              startAt={activeVideo.startAt}
+              onClose={() => setActiveVideo(null)}
+              onState={onState}
+              onOpenChannel={onOpenChannel}
+            />
+          </Suspense>
         )}
       </div>
     );
@@ -647,13 +650,15 @@ export default function Feed({
         />
       )}
       {activeVideo && (
-        <PlayerModal
-          video={activeVideo.video}
-          startAt={activeVideo.startAt}
-          onClose={() => setActiveVideo(null)}
-          onState={onState}
-          onOpenChannel={onOpenChannel}
-        />
+        <Suspense fallback={null}>
+          <PlayerModal
+            video={activeVideo.video}
+            startAt={activeVideo.startAt}
+            onClose={() => setActiveVideo(null)}
+            onState={onState}
+            onOpenChannel={onOpenChannel}
+          />
+        </Suspense>
       )}
       {isFetchingNextPage && (
         <div className="text-center text-muted py-4">{t("feed.loadingMore")}</div>