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.
Replace the Create React App frontend with a modern Vite stack. Adds a
login page (local form or SSO depending on auth mode), an OIDC callback
page, and a hosts page with create/edit/delete. Build output is emitted
into backend/embed/assets for embedding in the binary.
