fix(login): translated error messages for failed and rate-limited logins

Map INVALID_CREDENTIALS and TOO_MANY_ATTEMPTS to i18n keys. A non-JSON error body (Traefik 502/504) no longer shows as a network error.

public/i18n/de.json

@@ -11,7 +11,10 @@
     "SUBMIT": "Spiel betreten",
     "NO_ACCOUNT": "Noch kein Konto?",
     "ERROR_EMAIL": "Gib eine gültige Email-Adresse ein mit '@' und mit '.'.",
-    "ERROR_PASSWORD": "Das Passwort darf nicht leer sein."
+    "ERROR_PASSWORD": "Das Passwort darf nicht leer sein.",
+    "ERROR_INVALID_CREDENTIALS": "E-Mail oder Passwort ist falsch.",
+    "ERROR_TOO_MANY_ATTEMPTS": "Zu viele fehlgeschlagene Versuche. Bitte in {{minutes}} Minuten erneut versuchen.",
+    "ERROR_GENERIC": "Anmeldung fehlgeschlagen. Bitte später erneut versuchen."
   },
   "REGISTRATION": {
     "TITLE": "Registrieren",

public/i18n/en.json

@@ -11,7 +11,10 @@
     "SUBMIT": "Enter the game",
     "NO_ACCOUNT": "No account yet?",
     "ERROR_EMAIL": "Enter a valid email address with '@' and with '.'.",
-    "ERROR_PASSWORD": "The password must have at least one character."
+    "ERROR_PASSWORD": "The password must have at least one character.",
+    "ERROR_INVALID_CREDENTIALS": "Email or password is wrong.",
+    "ERROR_TOO_MANY_ATTEMPTS": "Too many failed attempts. Please try again in {{minutes}} minutes.",
+    "ERROR_GENERIC": "Login failed. Please try again later."
   },
   "REGISTRATION": {
     "TITLE": "Register",