proxy: isolate proxies from export renders and improve proxy workflow

- FootageJob::should_use_proxy() centralizes the proxy decision; worker
  pre-decode now honors the render mode so exports always decode the
  original media (previously every frame was pre-decoded from proxies)
- global Tools > Use Proxy Media toggle with footage invalidation
- ffmpeg -progress parsing for real percentage feedback while generating
- divider mode (1/2, 1/4, 1/8 of source resolution) with UI, proxy
  filename tags and per-footage persistence (pdivider)
- Media Offline warning slat rendered for missing footage
- regression tests: export isolation, relink invalidation, offline slat,
  progress parsing, divider arguments

app/dialog/proxy/proxydialog.h

@@ -43,6 +43,8 @@ public:
 
 	int proxy_height() const;
 
+	int proxy_divider() const;
+
 	int proxy_crf() const;
 
 	QString proxy_preset() const;
@@ -55,6 +57,8 @@ public:
 
 	void set_proxy_height(int height);
 
+	void set_proxy_divider(int divider);
+
 	void set_proxy_crf(int crf);
 
 	void set_proxy_preset(const QString &preset);
@@ -80,6 +84,8 @@ private:
 
 	IntegerSlider *height_slider_;
 
+	QComboBox *resolution_combo_;
+
 	IntegerSlider *crf_slider_;
 
 	QComboBox *preset_combo_;