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/codec/proxymanager.h

@@ -62,6 +62,11 @@ public:
 	struct ProxyParams {
 		int width = 1280;
 		int height = 720;
+		/**
+		 * @brief Source resolution divider (1 = use absolute width/height,
+		 * 2/4/8 = fraction of the source resolution)
+		 */
+		int divider = 1;
 		int version = 1;
 		QString extension = QStringLiteral("mp4");
 		int crf = 23;