feat: add devcontainer/nix support and refactor into modules

- Add .devcontainer/devcontainer.json for Podman development
- Add shell.nix for Nix users
- Refactor monolithic main.cpp into modular components:
  - parser: argument parsing
  - validator: configuration validation
  - converter: ffmpeg command building/execution
- Update Makefile with proper dependencies and test target
- Simplify build.sh to use make
- Update README with development environment docs

src/include/converter.hpp

@@ -0,0 +1,10 @@
+#ifndef CONVERTER_HPP
+#define CONVERTER_HPP
+
+#include "config.hpp"
+#include <string>
+
+std::string build_ffmpeg_command(const Config& config);
+int execute_conversion(const std::string& command, bool verbose);
+
+#endif // CONVERTER_HPP