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

shell.nix

@@ -0,0 +1,22 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  name = "davinci-video-converter";
+  
+  buildInputs = with pkgs; [
+    gcc13
+    make
+    ffmpeg
+    pkg-config
+  ];
+  
+  shellHook = ''
+    export CXX=g++
+    export CC=gcc
+    echo "DaVinci Video Converter development environment loaded"
+    echo "Available commands:"
+    echo "  make        - Build the project"
+    echo "  make clean  - Clean build artifacts"
+    echo "  make test   - Run tests"
+  '';
+}