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/render/renderprocessor.cpp

@@ -611,10 +611,23 @@ void RenderProcessor::ProcessAudioFootage(SampleBuffer &destination,
 		return;
 	}
 
+	// Mirror the video path: use the proxy (when enabled, ready, and containing
+	// audio) for offline renders only, never for export
+	const bool use_proxy =
+		static_cast<RenderMode::Mode>(ticket_->property("mode").toInt()) ==
+			RenderMode::kOffline &&
+		stream->has_proxy() && QFileInfo::exists(stream->proxy_filename());
+	const QString decode_filename = use_proxy ? stream->proxy_filename() :
+												stream->filename();
+	const QString decoder_id = use_proxy ? stream->proxy_decoder() :
+										   stream->decoder();
+	const int stream_index = use_proxy ?
+								 stream->proxy_stream_index() :
+								 stream->audio_params().stream_index();
+
 	DecoderPtr decoder = ResolveDecoderFromInput(
-		stream->decoder(),
-		Decoder::CodecStream(stream->filename(),
-							 stream->audio_params().stream_index(), nullptr));
+		decoder_id,
+		Decoder::CodecStream(decode_filename, stream_index, nullptr));
 
 	if (decoder) {
 		const AudioParams &audio_params = GetCacheAudioParams();