fix(frontend): localize avatar validation errors

checkAvatarFile returned hard-coded Russian strings, so an English user picking a
bad file got "Подойдёт PNG, JPG или WebP." with no translation. Route both
messages through the account namespace (validation.avatarType / avatarTooBig) the
same way the auth validators take a t function, keeping ru/en key parity enforced
by the satisfies clause.

frontend/src/features/account/i18n/en.ts

@@ -9,6 +9,10 @@ export const accountEn = {
   avatar: {
     change: 'Change avatar',
   },
+  validation: {
+    avatarType: 'PNG, JPG or WebP please.',
+    avatarTooBig: 'The file is over 5 MB.',
+  },
   devices: {
     heading: 'Linked games',
     linkedAt: 'Linked {{date}}',