为 Channel 模型新增 public_name 字段，让管理员可以为每个渠道
设置用户可见的友好名称（如"标准通道"、"高速通道"），替代前端
硬编码的"通道一/二/三"。Playground 通道选择器展示对外名称，
"渠道"统一改为"通道"。新建渠道时对外名称必填，编辑时可选。

Co-Authored-By: Claude <noreply@anthropic.com>

Add a new /api/user/channels endpoint that returns only enabled channels
with non-sensitive fields (id, name, type, remark) for token channel binding.
Update EditTokenModal to use this endpoint and display remark instead of ID
in the channel dropdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>