The backend could list/read/write a stopped server's world volume since the
file-editor slice, but the panel had no entry, so the one repair path for a
server that will not boot (a wrong line in server.properties) was API-only.
New /servers/:name/files page: breadcrumb browser, editor dialog with the
base64 []byte codec, binary files open read-only, the stopped gate is owned
up front (with a stop action) instead of letting every call 409, and a
doorway card on the console. i18n files namespace + wire-shape tests.