feat(auth): enhance password policy with stronger requirements

Update password validation across authentication screens to require minimum 8 characters with uppercase letters, lowercase letters, and numbers. Changes applied to ForgotPasswordScreen, RegisterStep3Profile, and AccountSecurityScreen. LoginScreen password validation was removed as it occurs post-authentication.

src/screens/profile/AccountSecurityScreen.tsx

@@ -149,8 +149,20 @@ export const AccountSecurityScreen: React.FC = () => {
       showPrompt({ title: '提示', message: '请输入邮箱验证码', type: 'warning' });
       return;
     }
-    if (newPassword.length < 6) {
-      showPrompt({ title: '提示', message: '新密码至少 6 位', type: 'warning' });
+    if (newPassword.length < 8) {
+      showPrompt({ title: '提示', message: '新密码至少 8 位', type: 'warning' });
+      return;
+    }
+    if (!/[A-Z]/.test(newPassword)) {
+      showPrompt({ title: '提示', message: '新密码必须包含大写字母', type: 'warning' });
+      return;
+    }
+    if (!/[a-z]/.test(newPassword)) {
+      showPrompt({ title: '提示', message: '新密码必须包含小写字母', type: 'warning' });
+      return;
+    }
+    if (!/[0-9]/.test(newPassword)) {
+      showPrompt({ title: '提示', message: '新密码必须包含数字', type: 'warning' });
       return;
     }
     if (newPassword !== confirmPassword) {
@@ -297,7 +309,7 @@ export const AccountSecurityScreen: React.FC = () => {
               <MaterialCommunityIcons name="lock-plus-outline" size={20} color={colors.text.secondary} style={styles.inputIcon} />
               <TextInput
                 style={styles.input}
-                placeholder="新密码（至少6位）"
+                placeholder="新密码（至少8位，含大小写字母和数字）"
                 placeholderTextColor={colors.text.hint}
                 value={newPassword}
                 onChangeText={setNewPassword}