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/web/ws.go

@@ -114,9 +114,9 @@ type wsHub struct {
 
 func newWSHub(auth *wsauth.Authenticator, devices *hub.Hub, log *logger.Logger) *wsHub {
 	h := &wsHub{
-		auth:    auth,
-		devices: devices,
-		log:     log,
+		auth:              auth,
+		devices:           devices,
+		log:               log,
 		clients:           make(map[*wsClient]struct{}),
 		screenCloseTimers: make(map[string]*time.Timer),
 	}
@@ -300,6 +300,23 @@ func (h *wsHub) OnResolutionChange(deviceID string, width, height int) {
 	}
 }
 
+// OnClipboardText relays a device's clipboard text back to every browser
+// watching it (in response to a "clipboard_get" request).
+func (h *wsHub) OnClipboardText(deviceID string, text string) {
+	msg := mustJSON(map[string]any{
+		"cmd":  "clipboard_text",
+		"id":   deviceID,
+		"text": text,
+	})
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	for c := range h.clients {
+		if c.watching == deviceID && c.token != "" {
+			c.queue(msg)
+		}
+	}
+}
+
 // OnScreenFrame ships a screen packet to every browser currently watching
 // this device. We hold the read lock for the whole iteration, but each
 // queueBinary is non-blocking (drops on backpressure) so a slow viewer