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.

CONTRIBUTING.md

@@ -0,0 +1,79 @@
+# Contributing
+
+> Project layout, build, and release flow. **If you only want to use the tool — read [README](README.md) instead.**
+
+## Repo layout
+
+```
+dhcpsrv/
+├── .github/
+│   ├── workflows/release.yml          ← CI: tag-driven build + GitHub Release
+│   └── ISSUE_TEMPLATE/                ← bug / feature / security routing
+├── src/dhcpsrv/                       ← package source (≤200 lines per module)
+│   ├── __init__.py                    ← single source of truth for __version__
+│   ├── __main__.py                    ← entry: python -m dhcpsrv
+│   ├── app.py                         ← main flow, wires everything
+│   ├── platform_win.py                ← VT enable + UAC self-elevate
+│   ├── update_check.py                ← GitHub /releases/latest poll
+│   ├── network.py                     ← list_adapters, netsh, ping (no shared state)
+│   ├── dhcp.py                        ← DhcpConfig, DhcpServer, packet parse/build
+│   └── ui.py                          ← rich-based full-screen TUI
+├── pyproject.toml                     ← deps, packaging, dynamic version
+├── CHANGELOG.md                       ← Keep a Changelog format, newest first
+├── CONTRIBUTING.md                    ← this file
+├── LICENSE / README.md / SECURITY.md
+└── .gitignore
+```
+
+## Run from source (no exe)
+
+```
+python -m pip install rich
+python -m dhcpsrv
+```
+
+`python -m dhcpsrv` finds `src/dhcpsrv/__main__.py` because the package lives under `src/`. You'll need administrator privileges for UDP/67 and `netsh` — the tool self-elevates via UAC.
+
+## Editable install (development)
+
+```
+python -m pip install -e .
+dhcpsrv
+```
+
+`-e .` makes the entry-point `dhcpsrv` available on PATH; edits in `src/dhcpsrv/` take effect immediately.
+
+## Build the portable .exe
+
+```
+python -m pip install pyinstaller rich
+python -m PyInstaller --onefile --uac-admin --console --name dhcpsrv --paths src dhcpsrv-launcher.py
+```
+
+`dhcpsrv-launcher.py` (at repo root) is the PyInstaller entry — it does an
+*absolute* import (`from dhcpsrv.app import main`) which is needed when
+PyInstaller runs the bundled script as a standalone module. The `--paths src`
+flag tells PyInstaller where to find the `dhcpsrv` package itself. Output:
+`dist/dhcpsrv.exe`.
+
+## Cut a release
+
+1. Update `src/dhcpsrv/__init__.py` — bump `__version__` to `X.Y.Z`.
+2. Update `CHANGELOG.md` — move items from `[Unreleased]` into a new `[X.Y.Z]` section with today's date.
+3. Commit: `git commit -am "vX.Y.Z: …"`.
+4. Tag: `git tag vX.Y.Z`.
+5. Push: `git push && git push --tags`.
+
+That's it. GitHub Actions picks up the tag, builds the exe, writes the SHA-256, and creates the GitHub Release with the zip attached.
+
+## Where features go
+
+| Adding... | Touch this module |
+|---|---|
+| A new DHCP option in the reply | `dhcp.py` → `DhcpServer.build_reply` |
+| A new adapter filter | `network.py` → `SKIP_DESCRIPTION` / `SKIP_MEDIA` / `list_adapters` |
+| A new column in the clients table | `ui.py` → `Ui._render_table` |
+| Something shown in the header | `ui.py` → `Ui._render_header` |
+| A startup check or banner line | `app.py` → `main()` |
+| A change to UAC / VT logic | `platform_win.py` |
+| Tweaking the GitHub update-check UX | `update_check.py` |