refactor(api): code-review fixes (R7 S1)

- localizeDetail: don't leak FastAPI's snake_case `loc` field names into a
  translated 422 toast; return a clean, fully localized validation message
  (drops the loc filter/pop parsing entirely)
- me(): drop the dead `r &&` guard now that req<T> types r non-nullable

frontend/src/i18n/locales/en/errors.json

@@ -1,7 +1,7 @@
 {
   "title": "Couldn't complete that",
   "generic": "The server couldn't carry out that action. Please try again.",
-  "validation": "Invalid value for “{{field}}”. Please check it and try again.",
+  "validation": "Some of the values you entered weren't valid. Please check them and try again.",
   "server": "Server error",
   "ok": "OK",
   "offline": {