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/widgets/window_buttons.dart

@@ -0,0 +1,133 @@
+import 'package:flutter/material.dart';
+import 'package:window_manager/window_manager.dart';
+
+import '../state/session_state.dart';
+
+class WindowButtons extends StatefulWidget {
+  final FocusNode? focusNode;
+  final bool fullscreen;
+  final VoidCallback? onToggleFullscreen;
+  const WindowButtons({
+    super.key,
+    this.focusNode,
+    this.fullscreen = false,
+    this.onToggleFullscreen,
+  });
+
+  @override
+  State<WindowButtons> createState() => _WindowButtonsState();
+}
+
+class _WindowButtonsState extends State<WindowButtons> with WindowListener {
+  bool _maximized = false;
+
+  @override
+  void initState() {
+    super.initState();
+    windowManager.addListener(this);
+    windowManager.isMaximized().then((v) {
+      if (mounted) setState(() => _maximized = v);
+    });
+  }
+
+  @override
+  void dispose() {
+    windowManager.removeListener(this);
+    super.dispose();
+  }
+
+  @override
+  void onWindowMaximize() {
+    if (mounted) setState(() => _maximized = true);
+  }
+
+  @override
+  void onWindowUnmaximize() {
+    if (mounted) setState(() => _maximized = false);
+  }
+
+  Future<void> _toggleMaximize() async {
+    if (widget.onToggleFullscreen != null) {
+      widget.onToggleFullscreen!();
+      return;
+    }
+    if (await windowManager.isMaximized()) {
+      await windowManager.unmaximize();
+    } else {
+      await windowManager.maximize();
+    }
+  }
+
+  Future<void> _close() async {
+    try {
+      await SessionState.instance.coordinator.leave();
+      await SessionState.instance.stopServer();
+    } catch (_) {}
+    await windowManager.close();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final focus = widget.focusNode;
+    final restored = widget.fullscreen || _maximized;
+    return Row(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        _WinBtn(
+          icon: Icons.remove,
+          tooltip: 'Minimize',
+          focusNode: focus,
+          onPressed: () => windowManager.minimize(),
+        ),
+        _WinBtn(
+          icon: restored ? Icons.filter_none : Icons.crop_square,
+          tooltip: restored ? 'Restore' : 'Maximize',
+          focusNode: focus,
+          onPressed: _toggleMaximize,
+        ),
+        _WinBtn(
+          icon: Icons.close,
+          tooltip: 'Close',
+          focusNode: focus,
+          hoverColor: Colors.red,
+          onPressed: _close,
+        ),
+      ],
+    );
+  }
+}
+
+class _WinBtn extends StatelessWidget {
+  final IconData icon;
+  final String tooltip;
+  final VoidCallback onPressed;
+  final FocusNode? focusNode;
+  final Color? hoverColor;
+  const _WinBtn({
+    required this.icon,
+    required this.tooltip,
+    required this.onPressed,
+    this.focusNode,
+    this.hoverColor,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return SizedBox(
+      width: 40,
+      height: 32,
+      child: IconButton(
+        focusNode: focusNode,
+        tooltip: tooltip,
+        padding: EdgeInsets.zero,
+        constraints: const BoxConstraints.tightFor(width: 40, height: 32),
+        iconSize: 15,
+        splashRadius: 12,
+        hoverColor: (hoverColor ?? Colors.white).withValues(alpha: 0.12),
+        highlightColor: Colors.transparent,
+        icon: Icon(icon, size: 15),
+        onPressed: onPressed,
+      ),
+    );
+  }
+}