fix(api,panel): refuse file operations on a server without a world volume (#58)

Live: the files page against a server whose world claim does not exist (never
started, or reaped) created a Job whose Pod stayed Pending on
FailedScheduling (persistentvolumeclaim not found) until the executor's 90s
wait expired — a 90s spinner answered by a misleading 504 files_timeout, for
a request that is knowably impossible. Backup and restore have refused this
shape with 409 no_world_volume since the #42 round; the file routes now run
the same gate before any Job is created, and the panel maps the code to a
localized message (it previously fell back to the English server text).

panel/src/i18n/resources/en-US/errors.json

@@ -14,6 +14,7 @@
   "console_unavailable": "Can't reach the server console right now — try again shortly.",
   "no_backup": "There's no restorable backup for this server yet.",
   "not_stopped": "Stop the server completely before restoring — a restore overwrites the live world volume.",
+  "no_world_volume": "This server has no world volume yet — start it once so it is created, then retry.",
   "restore_unavailable": "Restore isn't available right now — try again later.",
   "session_expired": "Your session expired — please sign in again.",
   "forbidden": "You are not allowed to do that.",