fix: Modal 弹窗高度约束，防止内容溢出视窗

- 添加 max-height 限制，确保弹窗始终在视窗内
- header/footer 固定不压缩，body 内容可滚动
- 兼容 iOS Safari 的 dvh 单位

src/styles/components/modal/_index.scss

@@ -25,6 +25,10 @@
     box-shadow: 0 8px 32px rgba(15, 23, 42, 0.16);
     width: 420px;
     max-width: 90vw;
+    max-height: calc(100vh - 48px);
+    max-height: calc(100dvh - 48px);
+    display: flex;
+    flex-direction: column;
     animation: modal-in 0.2s ease-out;
 }
 
@@ -47,6 +51,7 @@
     justify-content: space-between;
     padding: 16px 20px;
     border-bottom: 1px solid var(--color-border-2);
+    flex-shrink: 0;
 }
 
 .modal-title {
@@ -79,6 +84,9 @@
     font-size: $font-size-base;
     color: var(--color-text-2);
     line-height: 1.6;
+    flex: 1;
+    min-height: 0;
+    overflow-y: auto;
 }
 
 // Element: footer
@@ -89,4 +97,5 @@
     gap: 8px;
     padding: 12px 20px;
     border-top: 1px solid var(--color-border-2);
+    flex-shrink: 0;
 }