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/config.hpp

@@ -0,0 +1,15 @@
+#ifndef CONFIG_HPP
+#define CONFIG_HPP
+
+#include <string>
+
+struct Config {
+    std::string input_path;
+    std::string output_path;
+    std::string codec = "h264";
+    std::string quality = "medium";
+    int crf = 23;
+    bool verbose = false;
+};
+
+#endif // CONFIG_HPP