Sync Footage proxy state to render worker copy project

Proxy state (enabled/path/state/etc.) is stored as Footage member data,
not as a Node input, so ProjectCopier did not synchronize it to the
internal copy project used by worker processes. This caused toggling
Use Proxy in the UI to have no effect on rendered output.

Add a Footage::ProxySettingsChanged signal and have ProjectCopier sync
the proxy state to the copied Footage node, marking the copy project
modified so RenderWorkerPool writes a fresh graph snapshot.

app/node/project/footage/footage.cpp

@@ -240,6 +240,7 @@ void Footage::SetProxy(const QString &path,
 	proxy_video_stream_index_ = video_stream_index;
 	proxy_preset_version_ = preset_version;
 	proxy_enabled_ = enabled;
+	emit ProxySettingsChanged();
 }
 
 void Footage::ClearProxy()
@@ -249,6 +250,7 @@ void Footage::ClearProxy()
 	proxy_state_ = ProxyManager::kProxyMissing;
 	proxy_video_stream_index_ = -1;
 	proxy_preset_version_ = 0;
+	emit ProxySettingsChanged();
 }
 
 QString Footage::DescribeVideoStream(const VideoParams &params)

app/node/project/footage/footage.h

@@ -180,7 +180,10 @@ public:
 
 	void set_proxy_enabled(bool enabled)
 	{
-		proxy_enabled_ = enabled;
+		if (proxy_enabled_ != enabled) {
+			proxy_enabled_ = enabled;
+			emit ProxySettingsChanged();
+		}
 	}
 
 	const QString &proxy_path() const
@@ -240,6 +243,10 @@ public:
 							SerializedData *data) override;
 	virtual void SaveCustom(QXmlStreamWriter *writer) const override;
 
+signals:
+	void ProxySettingsChanged();
+
+public:
 	static const QString kFilenameInput;
 
 	virtual void AddedToGraphEvent(Project *p) override;