refactor: split into modules under src/dhcpsrv/

The previous monolithic dhcpsrv_app.py (~500 lines) is now 7 focused modules:

- src/dhcpsrv/__init__.py    : single source of truth for __version__
- src/dhcpsrv/__main__.py    : entry for python -m dhcpsrv
- src/dhcpsrv/app.py         : main orchestration, wires the rest
- src/dhcpsrv/platform_win.py: VT enable + UAC self-elevate
- src/dhcpsrv/update_check.py: GitHub /releases/latest poll
- src/dhcpsrv/network.py     : NIC enumeration, netsh, ping
- src/dhcpsrv/dhcp.py        : DhcpConfig, DhcpServer, packet parse/build, server loop
- src/dhcpsrv/ui.py          : rich-based full-screen TUI

Also added:
- dhcpsrv-launcher.py at repo root: absolute-import entry for PyInstaller
- pyproject.toml: deps + dynamic version
- CONTRIBUTING.md: layout, build, and release flow

CI workflow now builds from dhcpsrv-launcher.py.
No user-visible behaviour change.

dhcpsrv-launcher.py

@@ -0,0 +1,13 @@
+"""
+PyInstaller entry point — sits at the repo root and uses an *absolute* import
+so the bundled exe doesn't need relative-import resolution at runtime.
+
+For dev work without an install use `python -m dhcpsrv` instead (that path
+goes through `src/dhcpsrv/__main__.py` and relative imports work).
+"""
+
+from dhcpsrv.app import main
+
+
+if __name__ == "__main__":
+    main()