feat(auth): 手机号验证码登录（会话模型 + 登录页 + 导航接入）

core/model/src/main/kotlin/com/mingyuan/android_client/core/model/AuthModels.kt

@@ -0,0 +1,11 @@
+package com.mingyuan.android_client.core.model
+
+/**
+ * A logged-in session. Deliberately minimal for now: the token is the only thing needed to decide
+ * whether the user is logged in, and the user profile is still provided by [ProfileRepository].
+ * When the real backend lands, this grows a `user: AuthUser` field and the local data source grows
+ * a user cache.
+ */
+data class Session(
+    val token: String,
+)