Older Caddy versions don't support `list-modules --json`, which made the
plugins page fail with an opaque "exit status 1". Surface stderr in the error
and fall back to parsing the plain-text output (--packages / --skip-standard)
when --json is unsupported, so module detection works on any Caddy version.
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.
