Feature: Add configurable screen-preview quality levels menu

Add a "屏幕预览质量等级 (Screen Preview Quality Levels)" item to the
"高级 (Advanced)" menu that opens a dialog to edit the six preset
PreviewProfile levels (target encode width and JPEG quality) used by the
double-click single-preview and loop-snapshot floating windows. The dialog
edits a working copy in a full grid with per-field validation and a
restore-defaults button; on OK it persists the values to the [preview] INI
section.

ChooseScreenPreviewParams now reads from GetPreviewProfileCfg instead of the
hardcoded GetScreenPreviewProfile table, so preview requests sent after a
change use the configured parameters. LoadPreviewProfilesFromCfg loads the
saved levels at startup, falling back to the built-in defaults. Added
en_US/de_DE/zh_TW translations for the new menu and dialog strings.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/2015RemoteDlg.h

@@ -338,6 +338,15 @@ public:
     void SaveQualityProfilesToCfg() const;                // m_QualityProfiles 落盘到 THIS_CFG
     const QualityProfile& GetQualityProfileCfg(int level) const;  // 供 ScreenSpyDlg 下发用
     void OnQualityLevels();                               // 高级→远程桌面质量等级
+
+    // ===== 屏幕预览质量等级配置（6 档 PreviewProfile） =====
+    // 双击主机单发预览 / 循环快照浮窗所用。持久化走 THIS_CFG [preview] 节。
+    // 仅 UI 线程访问；修改后下次发预览请求即生效。
+    PreviewProfile m_PreviewProfiles[QUALITY_COUNT];
+    void LoadPreviewProfilesFromCfg();                    // 从 THIS_CFG 读到 m_PreviewProfiles（缺省回落 GetScreenPreviewProfile）
+    void SavePreviewProfilesToCfg() const;                // m_PreviewProfiles 落盘到 THIS_CFG
+    const PreviewProfile& GetPreviewProfileCfg(int level) const;  // 供 ChooseScreenPreviewParams 用
+    void OnPreviewLevels();                               // 高级→屏幕预览质量等级
     void SendThumbnailRequest(class context* ctx);// 发一次缩略图请求
     void CacheThumbnail(uint64_t clientID, const BYTE* jpeg, size_t bytes);
     void ClearThumbnailCacheEntry(uint64_t clientID);