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/DownloadCenter.tsx

@@ -1,4 +1,4 @@
-import { useMemo, useState } from "react";
+import { lazy, Suspense, useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -16,10 +16,12 @@ import {
 import clsx from "clsx";
 import Tabs, { usePersistedTab } from "./Tabs";
 import Modal from "./Modal";
-import DownloadDialog from "./DownloadDialog";
-import ProfileEditor from "./ProfileEditor";
-import VideoEditor from "./VideoEditor";
-import ShareDialog from "./ShareDialog";
+// Lazy: these dialogs/editors open on demand, so they stay out of the Downloads page chunk until
+// used (the video editor in particular is heavy — filmstrip + scrubber).
+const DownloadDialog = lazy(() => import("./DownloadDialog"));
+const ProfileEditor = lazy(() => import("./ProfileEditor"));
+const VideoEditor = lazy(() => import("./VideoEditor"));
+const ShareDialog = lazy(() => import("./ShareDialog"));
 import { useConfirm } from "./ConfirmProvider";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { api, type DownloadJob, type Me } from "../lib/api";
@@ -582,19 +584,21 @@ export default function DownloadCenter({ me }: { me: Me }) {
 
       {tab === "system" && me.role === "admin" && <AdminSystem />}
 
-      {addSource && (
-        <DownloadDialog
-          source={addSource}
-          onClose={() => {
-            setAddSource(null);
-            setAddUrl("");
-          }}
-        />
-      )}
-      {manage && <ProfileEditor onClose={() => setManage(false)} />}
-      {renameJob && <RenameModal job={renameJob} onClose={() => setRenameJob(null)} />}
-      {shareJob && <ShareDialog job={shareJob} onClose={() => setShareJob(null)} />}
-      {editJob && <VideoEditor job={editJob} onClose={() => setEditJob(null)} />}
+      <Suspense fallback={null}>
+        {addSource && (
+          <DownloadDialog
+            source={addSource}
+            onClose={() => {
+              setAddSource(null);
+              setAddUrl("");
+            }}
+          />
+        )}
+        {manage && <ProfileEditor onClose={() => setManage(false)} />}
+        {renameJob && <RenameModal job={renameJob} onClose={() => setRenameJob(null)} />}
+        {shareJob && <ShareDialog job={shareJob} onClose={() => setShareJob(null)} />}
+        {editJob && <VideoEditor job={editJob} onClose={() => setEditJob(null)} />}
+      </Suspense>
     </div>
   );
 }