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.

.github/workflows/release.yml

@@ -28,7 +28,7 @@ jobs:
         run: echo "version=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
 
       - name: Build executable
-        run: python -m PyInstaller --onefile --uac-admin --console --name dhcpsrv dhcpsrv_app.py
+        run: python -m PyInstaller --onefile --uac-admin --console --name dhcpsrv --paths src dhcpsrv-launcher.py
 
       - name: Package portable folder
         shell: pwsh