mobile(model-access): 手机版直连 model-access-server 长期方案跟踪

来自 PR makecindy/cindy-temp#36（登录后自动下发 XD 网关凭据）的规则 26 跟踪项。

**现状**：手机版本期零改动兼容——语音等链路经 voiceCredentialSync 穿透口自动携带桌面端同步下来的新凭据，功能不受影响。

**长期方案（本 issue 跟踪）**：手机版作为独立客户端直连 cindy-server 的 model-access-server，登录后自行拉取推理 endpoint + 用户专属 key，不再依赖桌面端穿透：

- 手机端接入 modelAccessApiBaseUrl 端点（clientEndpoints 字段已随 PR makecindy/cindy-temp#36 就位）；
- 凭据同步状态机（含 503 灰度关闭 / 403 企业未接入终态）在 mobile 侧的对应实现；
- 换号 / 登出时的凭据与模型目录清理。

关联：PR makecindy/cindy-temp#36、配套服务端 cindy-server(私有仓)#41。

Start by reviewing the existing clientEndpoints field and voiceCredentialSync behavior described in this issue, then read PR makecindy/cindy-temp#36 and the linked cindy-server issue #41. Define the mobile direct-connection flow, including 503 and 403 states plus account-switch and logout cleanup. Done means mobile can obtain its endpoint and user key without desktop credential passthrough.

Written by the indexing model from the issue text.