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/task/proxy/proxy.h

@@ -44,6 +44,16 @@ public:
 									  const ProxyManager::ProxyParams &params,
 									  const QString &output_filename);
 
+	/**
+	 * @brief Parses one line of ffmpeg "-progress" output
+	 *
+	 * Extracted for testability. If the line carries an output timestamp
+	 * ("out_time_us=" or "out_time_ms="), returns the progress fraction in
+	 * the range [0, 1] against duration_seconds. Returns a negative value
+	 * when the line carries no timestamp or duration_seconds is unknown.
+	 */
+	static double parse_progress(const QString &line, double duration_seconds);
+
 protected:
 	virtual bool run() override;