Desktop session batch 09-12: frameless chrome, bisect removal, chat/idle fixes

Frameless (window_manager, hidden titlebar): chrome bar on welcome,
drag-to-move appbar + window buttons on connect, drag layer + window
buttons in session controls; min size 160x90; volume-scroll off for
drag; session maximize = fullscreen toggle. Verified on KDE Wayland.
Delete legacy/minimal/nosheet bisect screens; session routes direct
(black-screen fix verified incl. WAN).
Avatar: pipeline now 128px aspect-kept PNG (was cropped JPEG);
profile card centered 64px hover-to-change; member avatar drops
ResizeImage downscale. NOTE open: pipeline 512 KiB check vs server
64 KiB enforcement mismatch.
Fixes: idle warn/unready skipped while PLAYING; sendChat drops
blanks; one chat row per send (drop textless server feed echo);
chat field keeps focus after send. Playback libass true; sdk floor
3.13. Handoff 9-11: 9+10 fixed, 11 jumpy seek still open.
Docs: handoff/roadmap/protocol record all of the above; few-commits
preference going forward.

apps/desktop/lib/main.dart

@@ -4,6 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:hive_ce_flutter/hive_flutter.dart';
 import 'package:media_kit/media_kit.dart';
 import 'package:sync_relay_core/core.dart';
+import 'package:window_manager/window_manager.dart';
 
 import 'app.dart';
 import 'src/state/session_state.dart';
@@ -32,6 +33,19 @@ Future<void> main(List<String> args) async {
         ),
       );
   WidgetsFlutterBinding.ensureInitialized();
+  await windowManager.ensureInitialized();
+  const windowOptions = WindowOptions(
+    size: Size(1280, 720),
+    minimumSize: Size(160, 90),
+    center: true,
+    backgroundColor: Color(0xFF000000),
+    titleBarStyle: TitleBarStyle.hidden,
+    title: 'Watch Together',
+  );
+  windowManager.waitUntilReadyToShow(windowOptions, () async {
+    await windowManager.show();
+    await windowManager.focus();
+  });
   await Hive.initFlutter();
   try {
     await SessionState.instance.loadForm();