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/pubspec.yaml

@@ -3,7 +3,7 @@ description: Watch-together desktop app (working title, repo sync_relay_project)
 publish_to: none
 version: 0.1.0
 environment:
-  sdk: '>=3.14.0-0 <4.0.0'
+  sdk: '>=3.13.0-0 <4.0.0'
 
 dependencies:
   flutter:
@@ -22,6 +22,7 @@ dependencies:
   hive_ce: ^2.19.3
   hive_ce_flutter: ^2.3.4
   image: ^4.9.2
+  window_manager: ^0.5.2
 dev_dependencies:
   flutter_test:
     sdk: flutter