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/src/sync/sync_client.dart

@@ -121,7 +121,12 @@ class SyncClient {
         } catch (_) {}
       case 'feed':
         try {
-          _feeds.add(FeedMessage.fromJson(decoded));
+          final f = FeedMessage.fromJson(decoded);
+          // Server delivers every chat twice: a ChatMessage carrying the
+          // text (converted to a feed entry in `case 'chat'` below) and a
+          // textless feed entry. Keep only the former: one send, one row,
+          // one toast, one sfx.
+          if (f.action != 'chat') _feeds.add(f);
         } catch (_) {}
       case 'kicked':
         try {