fix(desktop): 托盘菜单左缘对齐图标、去掉垂直间隙（12A 实测反馈）

此前菜单水平居中于托盘图标（观感偏左）且下方留 4px 间隙。改为
macOS 系统菜单习惯：左缘对齐图标左缘、紧贴菜单栏；菜单超出屏幕
右缘时整体左收 8px 边距。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

desktop/src-tauri/src/lib.rs

@@ -12,17 +12,30 @@ use tauri::tray::{MouseButton, MouseButtonState, TrayIconBuilder, TrayIconEvent}
 use tauri::Manager;
 use tauri_plugin_global_shortcut::{Shortcut, ShortcutState};
 
-/// 托盘图标 rect → 菜单窗口左上角物理坐标（图标正下方、水平居中）。
-fn tray_menu_position(rect: &tauri::Rect, scale: f64, win_width: f64) -> (f64, f64) {
+/// 托盘图标 rect → 菜单窗口左上角物理坐标。
+/// macOS 系统菜单习惯：左缘对齐图标左缘、紧贴菜单栏下方；
+/// 菜单超出屏幕右缘时整体左收（12A 实测反馈：居中定位偏左、间隙偏大）。
+fn tray_menu_position(
+    rect: &tauri::Rect,
+    scale: f64,
+    win_width: f64,
+    screen_right: Option<f64>,
+) -> (f64, f64) {
     let (x, y) = match rect.position {
         tauri::Position::Physical(p) => (p.x as f64, p.y as f64),
         tauri::Position::Logical(p) => (p.x * scale, p.y * scale),
     };
-    let (w, h) = match rect.size {
+    let (_w, h) = match rect.size {
         tauri::Size::Physical(s) => (s.width as f64, s.height as f64),
         tauri::Size::Logical(s) => (s.width * scale, s.height * scale),
     };
-    (x + w / 2.0 - win_width / 2.0, y + h + 4.0 * scale)
+    let mut mx = x;
+    if let Some(right) = screen_right {
+        if mx + win_width > right {
+            mx = right - win_width - 8.0 * scale;
+        }
+    }
+    (mx, y + h)
 }
 
 /// 左键点击托盘图标 → 在图标位置弹出 / 收起自绘菜单窗（11B）。
@@ -39,7 +52,10 @@ fn toggle_tray_menu(app: &tauri::AppHandle, rect: &tauri::Rect) {
         .outer_size()
         .map(|s| s.width as f64)
         .unwrap_or(248.0 * scale);
-    let (x, y) = tray_menu_position(rect, scale, win_width);
+    let screen_right = w.current_monitor().ok().flatten().map(|m| {
+        m.position().x as f64 + m.size().width as f64
+    });
+    let (x, y) = tray_menu_position(rect, scale, win_width, screen_right);
     let _ = w.set_position(tauri::PhysicalPosition::new(x, y));
     let _ = w.show();
     let _ = w.set_focus(); // 获焦后才能在失焦时自动收起