Add unit/integration tests across backend handlers, auth, caddy and
util packages, plus frontend component and API tests with the new
test setup. Update CI workflow and Vite config to run the suites.
Introduce per-host plugin support: Caddy module discovery, module
config endpoints, plugin schemas (basicauth, cloudflare) and the
host plugins API. On the frontend, add the Plugins page, plugin/host
plugin dialogs and schema-driven form fields.
