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

.devcontainer/devcontainer.json

@@ -0,0 +1,41 @@
+{
+  "name": "DaVinci Video Converter",
+  "image": "docker.io/library/fedora:39",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-vscode.cpptools",
+        "ms-vscode.cmake-tools",
+        "twxs.cmake"
+      ],
+      "settings": {
+        "terminal.integrated.defaultProfile.linux": "bash",
+        "C_Cpp.default.compilerPath": "/usr/bin/g++",
+        "C_Cpp.default.cStandard": "gnu17",
+        "C_Cpp.default.cppStandard": "gnu++17"
+      }
+    }
+  },
+  "features": {
+    "ghcr.io/devcontainers/features/common-utils:2": {
+      "installZsh": true,
+      "configureZshAsDefaultShell": true,
+      "installOhMyZsh": true,
+      "upgradePackages": true,
+      "username": "vscode",
+      "userUid": "1000",
+      "userGid": "1000"
+    }
+  },
+  "postCreateCommand": "dnf install -y gcc-c++ make ffmpeg ffmpeg-devel && dnf clean all",
+  "remoteUser": "vscode",
+  "updateRemoteUserUID": true,
+  "mounts": [
+    "source=${localEnv:HOME}${localEnv:USERPROFILE}/.ssh,target=/home/vscode/.ssh,type=bind,consistency=cached"
+  ],
+  "runArgs": [
+    "--cap-add=SYS_PTRACE",
+    "--security-opt",
+    "seccomp=unconfined"
+  ]
+}