proxy: per-footage presets, audio in proxies, configurable ffmpeg path

- Footage can store custom proxy parameters (width/height/crf/preset/
  extension/audio) that override the global settings; they are
  serialized with the project and used by every generation entry point
  via Footage::GetEffectiveProxyParams()
- Proxies now include the source audio streams (AAC) unless disabled;
  the proxy filename records the audio flag and offline audio rendering
  decodes from the proxy when present
- ProxyTask resolves ffmpeg from the new FFmpegPath config key first,
  then PATH, then common install locations (e.g. Homebrew on macOS),
  instead of relying on PATH only; the error message points at the
  preferences when no executable is found
- ProxyTask::BuildArguments() is extracted for testability
- Add ProxyIncludeAudio and FFmpegPath config defaults
- Add regression tests for the filename audio marker, config-backed
  params, ffmpeg resolution, argument building, and custom-param
  persistence

app/task/proxy/proxy.h

@@ -32,6 +32,18 @@ public:
 			  const ProxyManager::ProxyParams &params,
 			  const QString &output_filename);
 
+	/**
+	 * @brief Builds the ffmpeg command line for a proxy generation run
+	 *
+	 * Extracted for testability. The video stream is always mapped first so
+	 * that it is stream 0 in the proxy file; audio streams (when enabled)
+	 * follow in source order.
+	 */
+	static QStringList BuildArguments(const QString &source_filename,
+									  int stream_index,
+									  const ProxyManager::ProxyParams &params,
+									  const QString &output_filename);
+
 protected:
 	virtual bool Run() override;