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_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