Make settings window independent and taskbar-aware

Convert the settings window into an independent top-level window with its own taskbar icon and open-or-focus semantics. Removed Owner/anchor/toggle semantics from SettingsWindowService and added ScreenReferenceWindow for centering; settings windows now ShowInTaskbar = true and are truly destroyed on close. Added SettingsWindowPlacementHelper and tests for placement/centering. Main window now respects an AppSettingsSnapshot.ShowInTaskbar flag (new setting exposed in GeneralSettings UI) and slide/visibility animations and "back to Windows" behavior no longer affect the independent settings window. Updated various callers to use OpenIndependentSettingsModule, adjusted window transitions/X offsets, and added/updated spec files documenting the feature and animation boundary.

.trae/specs/independent-settings-window/tasks.md

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+# Tasks
+
+- [x] Task 1: 简化设置窗口打开契约
+  - [x] 将 `SettingsWindowOpenRequest` 从 owner / anchor 语义改为目标页 + 参考屏幕语义
+  - [x] 移除 `ISettingsWindowService.Toggle`
+
+- [x] Task 2: 重做设置窗口服务行为
+  - [x] 设置窗口始终使用 `Show()` 打开
+  - [x] 设置窗口始终 `ShowInTaskbar = true`
+  - [x] 已打开时只聚焦并在需要时切页
+  - [x] 关闭后销毁实例，下次打开重新创建并居中
+
+- [x] Task 3: 统一设置入口并解耦桌面壳
+  - [x] 桌面底栏设置按钮改为 open-or-focus
+  - [x] 组件库入口改为复用 `OpenIndependentSettingsModule`
+  - [x] 移除 `MainWindow` 上的设置窗口锚点逻辑
+
+- [x] Task 4: 明确产品边界
+  - [x] 调整“在任务栏显示图标”文案，限定为桌面主窗口
+  - [x] 新增独立设置窗口 feature spec
+  - [x] 在窗口过渡动画 spec 中补充“设置窗口不参与动画”
+
+- [x] Task 5: 验证
+  - [x] 运行 `dotnet build LanMountainDesktop.slnx -c Debug`
+  - [x] 运行与新 helper 相关的测试