fix: add running guards to prevent concurrent toggleProcess/exitNodeProcess

Rapid clicks or slow ops no longer spawn duplicate tailscale invocations.
Matches existing pattern used for copyProcess safety.
Fixes #15

tailscalectl/TailscaleWidget.qml

@@ -114,12 +114,14 @@ PluginComponent {
     }
 
     function toggleTailscale() {
+        if (toggleProcess.running) return
         root._pendingToggleAction = root.isConnected ? "disconnect" : "connect"
         toggleProcess.command = TailscaleLib.buildToggleCommand(root.isConnected)
         toggleProcess.running = true
     }
 
     function setExitNode(hostname) {
+        if (exitNodeProcess.running) return
         exitNodeProcess.command = TailscaleLib.makeExitNodeCommand(hostname)
         exitNodeProcess.running = true
     }