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/linux/flutter/generated_plugin_registrant.cc

@@ -8,6 +8,8 @@
 
 #include <media_kit_libs_linux/media_kit_libs_linux_plugin.h>
 #include <media_kit_video/media_kit_video_plugin.h>
+#include <screen_retriever_linux/screen_retriever_linux_plugin.h>
+#include <window_manager/window_manager_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
   g_autoptr(FlPluginRegistrar) media_kit_libs_linux_registrar =
@@ -16,4 +18,10 @@ void fl_register_plugins(FlPluginRegistry* registry) {
   g_autoptr(FlPluginRegistrar) media_kit_video_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "MediaKitVideoPlugin");
   media_kit_video_plugin_register_with_registrar(media_kit_video_registrar);
+  g_autoptr(FlPluginRegistrar) screen_retriever_linux_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "ScreenRetrieverLinuxPlugin");
+  screen_retriever_linux_plugin_register_with_registrar(screen_retriever_linux_registrar);
+  g_autoptr(FlPluginRegistrar) window_manager_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "WindowManagerPlugin");
+  window_manager_plugin_register_with_registrar(window_manager_registrar);
 }

apps/desktop/linux/flutter/generated_plugins.cmake

@@ -5,6 +5,8 @@
 list(APPEND FLUTTER_PLUGIN_LIST
   media_kit_libs_linux
   media_kit_video
+  screen_retriever_linux
+  window_manager
 )
 
 list(APPEND FLUTTER_FFI_PLUGIN_LIST