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/config/config.cpp

@@ -235,6 +235,10 @@ void Config::SetDefaults()
 	SetEntryInternal(QStringLiteral("ProxyCRF"), NodeValue::kInt, 23);
 	SetEntryInternal(QStringLiteral("ProxyPreset"), NodeValue::kText,
 					 QStringLiteral("veryfast"));
+	SetEntryInternal(QStringLiteral("ProxyIncludeAudio"), NodeValue::kBoolean,
+					 true);
+	SetEntryInternal(QStringLiteral("FFmpegPath"), NodeValue::kText,
+					 QString());
 
 	SetEntryInternal(QStringLiteral("DefaultSequenceWidth"), NodeValue::kInt,
 					 1920);