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.
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.
