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/go/cmd/main.go

@@ -135,6 +135,8 @@ func (h *MyHandler) OnReceive(ctx *connection.Context, data []byte) {
 		// arrives shortly after via TOKEN_NEXTSCREEN.
 	case protocol.TokenNextScreen:
 		h.handleScreenFrame(ctx, data, false)
+	case protocol.TokenClipboardText:
+		h.handleClipboardText(ctx, data)
 	case protocol.TokenKeyframe:
 		// Sent by the client only when frameID % m_GOP == 0; the client's
 		// DEFAULT_GOP is 0x7FFFFFFF (effectively infinite), so this token
@@ -427,6 +429,18 @@ func (h *MyHandler) handleScreenFrame(ctx *connection.Context, data []byte, alwa
 	h.hub.PublishScreenFrame(deviceID, packet, isKey)
 }
 
+// handleClipboardText handles TOKEN_CLIPBOARD_TEXT (118): the device replies to
+// a COMMAND_SCREEN_GET_CLIPBOARD request with its clipboard text as
+// [118][UTF-8 text][NUL]. data[0] is the token; the payload is data[1:].
+func (h *MyHandler) handleClipboardText(ctx *connection.Context, data []byte) {
+	deviceID := h.hub.ScreenDeviceID(ctx)
+	if deviceID == "" {
+		return // not a bound screen sub-conn — drop
+	}
+	text := strings.TrimRight(string(data[1:]), "\x00")
+	h.hub.PublishClipboardText(deviceID, text)
+}
+
 // handleLogin handles client login (TOKEN_LOGIN = 102)
 func (h *MyHandler) handleLogin(ctx *connection.Context, data []byte) {
 	info, err := protocol.ParseLoginInfo(data)