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

core/domain/src/main/kotlin/com/mingyuan/android_client/core/domain/repository/SessionRepository.kt

@@ -1,11 +1,30 @@
 package com.mingyuan.android_client.core.domain.repository
 
+import com.mingyuan.android_client.core.model.Session
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.map
+
+sealed interface SmsCodeResult {
+    data object Success : SmsCodeResult
+    data class Failure(val message: String) : SmsCodeResult
+}
+
+sealed interface LoginResult {
+    data object Success : LoginResult
+    data class Failure(val message: String) : LoginResult
+}
 
 interface SessionRepository {
-    val isLoggedIn: Flow<Boolean>
+    /** The current session, or `null` when logged out. */
+    val session: Flow<Session?>
 
-    suspend fun login()
+    /** Derived convenience kept for the existing feature ViewModels (home/checkin/plan/profile). */
+    val isLoggedIn: Flow<Boolean>
+        get() = session.map { it != null }
+
+    suspend fun requestSmsCode(phone: String): SmsCodeResult
+
+    suspend fun login(phone: String, code: String): LoginResult
 
     suspend fun logout()
 }