Create UserAgentClient (http.BaseClient wrapper) that injects a
User-Agent header into every request, reading app name and version
from VersionService and contact/homepage from dev_config.dart.
Format follows OSM tile usage policy:
DeFlock/<version> (+https://deflock.org; contact: admin@stopflock.com)
Replaces 4 inconsistent hardcoded UA strings and adds UA to the 9
call sites that previously sent none.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
