Protocol 15: one shared session log on the server, chat never logged, Mark a problem

Every app streams its redacted diagnostic lines (server time, batched
every 10 s, gzip, buffered while offline) to POST /l/; the server merges
them with its own into session-YYYY-MM-DD.log in time order, tagged by
who. Opt-out in Settings > About; Mark a problem (Ctrl+Shift+B, the
player menus, About) writes a marker with an optional note. Copy
diagnostics can include the server's session log. Chat text never
reaches any log.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/shell/session_player.dart

@@ -29,6 +29,7 @@ import '../widgets/app_keys.dart';
 import '../widgets/text_focus.dart';
 import '../widgets/window_buttons.dart';
 import 'avatar.dart';
+import 'mark_problem.dart';
 import 'motion.dart';
 import 'now_playing_title.dart';
 import 'player_status.dart';
@@ -627,6 +628,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
             (title: 'Change video', hint: null, run: widget.onChangeVideo),
             (title: 'Chat & activity', hint: 'Tab', run: _toggleChat),
             (title: 'Reload video', hint: 'Ctrl+R', run: _reloadVideo),
+            (title: 'Mark a problem', hint: 'Ctrl+Shift+B', run: () => showMarkProblem(context)),
             (title: 'Mark everyone ready', hint: null, run: coord.forcePlayAll),
             (title: 'Fullscreen', hint: 'F', run: widget.onToggleFullscreen),
             (title: 'Leave server', hint: 'Esc', run: widget.onLeave),