auth: 邮箱注册/登录补全 Linux 账号创建

registerByEmail / loginByEmail 漏掉 ensureLinuxAccount，开 CLAW_ENABLE_LINUX_ACCOUNTS=1
时邮箱用户既不会建 Linux 账号、createAccountDirectories 的 chown 也会失败。
照搬 username 路径的位置补上即可。同时删掉后端 server.py 里同逻辑的 dead code
（_XIAOMI_EMAIL_RE / _account_id_from_email），实际拆分发生在 TS 层。

backend/api/server.py

@@ -4556,28 +4556,6 @@ def _safe_account_id(account_id: str | None) -> str:
     return normalized[:80] or 'default'
 
 
-_XIAOMI_EMAIL_RE = re.compile(r'^([\w.-]+)@xiaomi\.com$', re.IGNORECASE)
-_EMAIL_ACCOUNT_ID_RE = re.compile(r'^[a-z0-9._-]{2,40}$')
-
-
-def _account_id_from_email(email: str | None) -> str | None:
-    """Validate a Xiaomi email and return its prefix as account_id.
-
-    The prefix is lowercased and must satisfy the platform account_id rule:
-    only lowercase letters / digits / dot / underscore / dash, length 2-40.
-    Returns None if the email or prefix is invalid (caller must reject).
-    """
-    if not isinstance(email, str):
-        return None
-    match = _XIAOMI_EMAIL_RE.match(email.strip())
-    if not match:
-        return None
-    prefix = match.group(1).lower()
-    if not _EMAIL_ACCOUNT_ID_RE.match(prefix):
-        return None
-    return prefix
-
-
 def _linux_accounts_enabled() -> bool:
     return os.environ.get(LINUX_ACCOUNT_ENV, '').strip().lower() in {
         '1',