Feature: Add monitor-switch buttons to remote toolbars (#15314)

* Feature: add monitor-switch buttons to remote toolbars

Add a one-click "switch to next monitor" control to both desktop toolbars:
- Main toolbar: always shown when the remote has more than one monitor,
  styled to match the existing blue icon buttons (white screen, black number).
- Minimized (draggable show/hide) toolbar: off by default, toggled via a new
  "Show monitor switch on minimized toolbar" checkbox in the Display menu and
  persisted as a local option.

Signed-off-by: StealUrKill <35749471+StealUrKill@users.noreply.github.com>

* Update remote_toolbar.dart

* refact: unify monitor-switch button icons, share tooltip

Addressing the review feedback on the monitor-switch toolbar buttons:

- Add assets/display_switcher.svg and use it for both the main and minimized buttons. This replaces the hand-drawn glyph (Containers + magic numbers) on the main toolbar. The icon scales with DPI/theme and the two toolbars stay visually consistent.
- Flip the minimized button's label to white for contrast, since the new icon has a solid screen.
- Move the tooltip string into a shared _MonitorCycle.tooltip getter so both buttons use one source of truth.
- Use const Offstage() for consistency with the surrounding returns.

Signed-off-by: StealUrKill <35749471+StealUrKill@users.noreply.github.com>

* Improve monitor-switch settings and toolbar behavior

- Nest the minimized-toolbar option under the main one in settings only show when the main option is enabled.

- Only show the minimized switch button on the collapsed toolbar handle, so it no longer duplicates the main switch while the toolbar is expanded.

Signed-off-by: StealUrKill <35749471+StealUrKill@users.noreply.github.com>

---------

Signed-off-by: StealUrKill <35749471+StealUrKill@users.noreply.github.com>
Co-authored-by: RustDesk <71636191+rustdesk@users.noreply.github.com>

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -407,6 +407,7 @@ class _GeneralState extends State<_General> {
   final RxBool serviceStop =
       isWeb ? RxBool(false) : Get.find<RxBool>(tag: 'stop-service');
   RxBool serviceBtnEnabled = true.obs;
+  final GlobalKey _minToolbarOptionKey = GlobalKey();
 
   @override
   Widget build(BuildContext context) {
@@ -605,6 +606,47 @@ class _GeneralState extends State<_General> {
         },
       ));
     }
+    children.add(_OptionCheckBox(
+      context,
+      'Show monitor switch button on the main toolbar',
+      kOptionAllowMonitorSwitchMainToolbar,
+      isServer: false,
+      update: (enabled) async {
+        if (!enabled) {
+          await mainSetLocalBoolOption(
+              kOptionAllowMonitorSwitchMinToolbar, false);
+        }
+        if (mounted) setState(() {});
+        reloadAllWindows();
+        if (enabled) {
+          WidgetsBinding.instance.addPostFrameCallback((_) {
+            final ctx = _minToolbarOptionKey.currentContext;
+            if (ctx != null) {
+              Scrollable.ensureVisible(
+                ctx,
+                alignment: 0.5,
+                duration: const Duration(milliseconds: 250),
+                curve: Curves.easeInOut,
+              );
+            }
+          });
+        }
+      },
+    ));
+    if (mainGetLocalBoolOptionSync(kOptionAllowMonitorSwitchMainToolbar)) {
+      children.add(KeyedSubtree(
+        key: _minToolbarOptionKey,
+        child: _OptionCheckBox(
+          context,
+          'Show on the minimized toolbar',
+          kOptionAllowMonitorSwitchMinToolbar,
+          isServer: false,
+          update: (_) {
+            reloadAllWindows();
+          },
+        ).marginOnly(left: _kCheckBoxLeftMargin * 3),
+      ));
+    }
     return _Card(title: 'Other', children: children);
   }