Refactor: Use common password validator

Refactors password validation logic to use a common `PasswordValidator` utility. This ensures consistent password validation rules across different parts of the application, including:

- User login
- Email sign-up
- Password change
- Account removal

Also adds a string resource for password too long error message.

app/src/main/java/com/aiosman/ravenow/utils/PasswordValidator.kt

@@ -0,0 +1,125 @@
+package com.aiosman.ravenow.utils
+
+import android.content.Context
+import com.aiosman.ravenow.R
+
+/**
+ * 密码校验工具类
+ * 提供统一的密码校验规则和错误信息
+ */
+object PasswordValidator {
+    
+    /**
+     * 密码最大长度限制
+     */
+    const val MAX_PASSWORD_LENGTH = 64
+    
+    /**
+     * 密码最小长度限制
+     */
+    const val MIN_PASSWORD_LENGTH = 6
+    
+    /**
+     * 密码校验结果
+     */
+    data class ValidationResult(
+        val isValid: Boolean,
+        val errorMessage: String? = null
+    )
+    
+    /**
+     * 校验密码格式
+     * @param password 密码
+     * @param context 上下文，用于获取本地化错误信息
+     * @return 校验结果
+     */
+    fun validatePassword(password: String, context: Context): ValidationResult {
+        return when {
+            // 检查是否为空
+            password.isEmpty() -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_required)
+            )
+            
+            // 检查长度不能超过64个字符
+            password.length > MAX_PASSWORD_LENGTH -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_too_long, MAX_PASSWORD_LENGTH)
+            )
+            
+            // 检查最小长度
+            password.length < MIN_PASSWORD_LENGTH -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_format)
+            )
+            
+            // 检查是否包含至少一个数字
+            !password.matches(Regex(".*\\d.*")) -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_format)
+            )
+            
+            // 检查是否包含字母
+            !password.matches(Regex(".*[a-zA-Z].*")) -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_format)
+            )
+            
+            else -> ValidationResult(true)
+        }
+    }
+    
+    /**
+     * 校验密码确认
+     * @param password 原密码
+     * @param confirmPassword 确认密码
+     * @param context 上下文
+     * @return 校验结果
+     */
+    fun validatePasswordConfirmation(
+        password: String,
+        confirmPassword: String,
+        context: Context
+    ): ValidationResult {
+        return when {
+            confirmPassword.isEmpty() -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_confirm_password_required)
+            )
+            
+            confirmPassword.length > MAX_PASSWORD_LENGTH -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_too_long, MAX_PASSWORD_LENGTH)
+            )
+            
+            password != confirmPassword -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_confirm_password_mismatch)
+            )
+            
+            else -> ValidationResult(true)
+        }
+    }
+    
+    /**
+     * 校验当前密码（用于更改密码和删除账户场景）
+     * @param currentPassword 当前密码
+     * @param context 上下文
+     * @return 校验结果
+     */
+    fun validateCurrentPassword(currentPassword: String, context: Context): ValidationResult {
+        return when {
+            currentPassword.isEmpty() -> ValidationResult(
+                false,
+                "Please enter your current password"
+            )
+            
+            currentPassword.length > MAX_PASSWORD_LENGTH -> ValidationResult(
+                false,
+                context.getString(R.string.text_error_password_too_long, MAX_PASSWORD_LENGTH)
+            )
+            
+            else -> ValidationResult(true)
+        }
+    }
+}