Feature: Web remote desktop bidirectional text clipboard sync

Add keyboard-shortcut (Ctrl+C / Ctrl+X / Ctrl+V) text clipboard transfer
between the browser and the remote desktop, reusing the clipboard protocol
that already exists in the native viewer (COMMAND_SCREEN_GET_CLIPBOARD=24,
COMMAND_SCREEN_SET_CLIPBOARD=25, TOKEN_CLIPBOARD_TEXT=118) so the client
needs no change.

Local -> remote: pressing Ctrl+V reads the browser clipboard, pushes it to
the remote clipboard (UTF-8 -> GBK, matching the client's CF_TEXT/ANSI
handling) and then synthesizes a remote Ctrl+V so the pasted text is the
newly delivered one.

Remote -> local: Ctrl+C or Ctrl+X is forwarded to the remote as usual and,
after a short delay, the remote clipboard is pulled back and written to the
browser clipboard for a later local Ctrl+V.

Both the MFC and Go servers get the same three clipboard signaling paths
(clipboard_set, clipboard_get, clipboard_text) so the shared single-page
frontend behaves identically on either backend.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/WebService.h

@@ -160,6 +160,9 @@ public:
     // Cursor change notification (called from ScreenSpyDlg)
     void BroadcastCursor(uint64_t device_id, uint8_t cursor_index);
 
+    // 远程剪贴板文本回传通知（called from ScreenSpyDlg，payload 为 UTF-8）
+    void NotifyClipboardText(uint64_t device_id, const std::string& text);
+
     // Get count of web clients watching a device
     int GetWebClientCount(uint64_t device_id);
 
@@ -187,6 +190,8 @@ private:
     void HandlePing(void* ws_ptr, const std::string& token);
     void HandleMouse(void* ws_ptr, const std::string& msg);
     void HandleKey(void* ws_ptr, const std::string& msg);
+    void HandleClipboardSet(void* ws_ptr, const std::string& msg);
+    void HandleClipboardGet(void* ws_ptr, const std::string& token);
     void HandleRdpReset(void* ws_ptr, const std::string& token);
     void HandleAudioToggle(void* ws_ptr, const std::string& token);