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/codec/proxymanager.h

@@ -66,6 +66,7 @@ public:
 		QString extension = QStringLiteral("mp4");
 		int crf = 23;
 		QString preset = QStringLiteral("veryfast");
+		bool include_audio = true;
 	};
 
 	struct Proxy {
@@ -89,6 +90,27 @@ public:
 
 	static ProxyState ProxyStateFromString(const QString &state);
 
+	/**
+	 * @brief Returns true if a proxy filename generated by GetProxyFilename()
+	 * indicates the proxy contains audio streams
+	 */
+	static bool ProxyFilenameHasAudio(const QString &proxy_filename);
+
+	/**
+	 * @brief Builds proxy parameters from the global application config
+	 */
+	static ProxyParams ProxyParamsFromConfig();
+
+	/**
+	 * @brief Locates an ffmpeg executable for proxy generation
+	 *
+	 * Resolution order: the explicitly configured path (if non-empty and an
+	 * existing executable file), then the system PATH, then common
+	 * platform-specific install locations. Returns an empty string if no
+	 * executable could be found.
+	 */
+	static QString FindFFmpegExecutable(const QString &configured_path);
+
 	Proxy GetOrStartProxy(const QString &cache_path,
 						  const QString &source_filename, int stream_index,
 						  const ProxyParams &params);