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/app.dart

@@ -1,25 +1,11 @@
-import 'dart:io';
-
 import 'package:flutter/material.dart';
 import 'package:signals/signals_flutter.dart';
 
 import 'src/screens/connect_screen.dart';
-import 'src/screens/legacy_session_screen.dart';
-import 'src/screens/minimal_session_screen.dart';
-import 'src/screens/nosheet_session_screen.dart';
 import 'src/screens/session_screen.dart';
 import 'src/screens/welcome_screen.dart';
 import 'src/state/session_state.dart';
 
-bool get useLegacySession =>
-    Platform.environment['SYNC_RELAY_UI'] == 'legacy';
-
-bool get useMinimalSession =>
-    Platform.environment['SYNC_RELAY_UI'] == 'minimal';
-
-bool get useNosheetSession =>
-    Platform.environment['SYNC_RELAY_UI'] == 'nosheet';
-
 class WatchTogetherApp extends StatelessWidget {
   const WatchTogetherApp({super.key});
 
@@ -34,13 +20,7 @@ class WatchTogetherApp extends StatelessWidget {
           return switch (session.route.value) {
             AppRoute.welcome => const WelcomeScreen(),
             AppRoute.connect => const ConnectScreen(),
-            AppRoute.session => useMinimalSession
-                ? const MinimalSessionScreen()
-                : useNosheetSession
-                    ? const NosheetSessionScreen()
-                    : useLegacySession
-                        ? const LegacySessionScreen()
-                        : const SessionScreen(),
+            AppRoute.session => const SessionScreen(),
           };
         },
       ),