feat(client): add GiteaClient with auth and pagination (fixes #1)

- GiteaClient with requests.Session and token auth header
- _get_paginated for automatic pagination (limit=50)
- get_repos, get_latest_release (None on 404), get_milestones
- 9 unit tests with mocked requests.Session
- Fix setuptools build backend in pyproject.toml

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=68.0", "wheel"]
-build-backend = "setuptools.backends._legacy:_Backend"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "gitea-dashboard"