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/windows/flutter/generated_plugin_registrant.cc

@@ -8,10 +8,16 @@
 
 #include <media_kit_libs_windows_video/media_kit_libs_windows_video_plugin_c_api.h>
 #include <media_kit_video/media_kit_video_plugin_c_api.h>
+#include <screen_retriever_windows/screen_retriever_windows_plugin_c_api.h>
+#include <window_manager/window_manager_plugin.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
   MediaKitLibsWindowsVideoPluginCApiRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("MediaKitLibsWindowsVideoPluginCApi"));
   MediaKitVideoPluginCApiRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("MediaKitVideoPluginCApi"));
+  ScreenRetrieverWindowsPluginCApiRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("ScreenRetrieverWindowsPluginCApi"));
+  WindowManagerPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("WindowManagerPlugin"));
 }