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/components/AvatarUpload.tsx

@@ -19,7 +19,7 @@ export function AvatarUpload() {
     const file = e.target.files?.[0]
     e.target.value = ''
     if (!file) return
-    const problem = checkAvatarFile(file)
+    const problem = checkAvatarFile(file, t)
     if (problem) {
       setError(problem)
       return