feat: 滚动时 banner 动态压缩释放更多内容空间

- header 改为 flex 布局驱动，滚动超过 60px 自动压缩为紧凑模式
- 压缩态：标题字号缩小、副标题隐藏、添加底部阴影
- body 改为 flex column 布局，去掉硬编码高度计算
- 去除 ResizeObserver 反馈循环，消除滚动抖动

Co-Authored-By: Claude Code <noreply@anthropic.com>

index.html

@@ -31,17 +31,42 @@
   --type-scenario: #a855f7;
 }
 * { margin: 0; padding: 0; box-sizing: border-box; }
-body { font-family: -apple-system, 'Segoe UI', sans-serif; background: var(--bg); color: var(--text); line-height: 1.6; min-height: 100vh; }
+body { font-family: -apple-system, 'Segoe UI', sans-serif; background: var(--bg); color: var(--text); line-height: 1.6; height: 100vh; display: flex; flex-direction: column; overflow: hidden; }
 
 /* Header */
-.header { background: var(--header-bg); padding: 28px 24px; text-align: center; border-bottom: 1px solid var(--surface2); position: relative; }
-.header h1 { font-size: 26px; font-weight: 700; margin-bottom: 6px; }
-.header p { color: rgba(255,255,255,0.7); font-size: 14px; }
-.theme-toggle { position: absolute; top: 16px; right: 16px; background: rgba(255,255,255,0.15); border: none; color: white; width: 36px; height: 36px; border-radius: 8px; cursor: pointer; font-size: 18px; transition: background 0.2s; }
+.header {
+  background: var(--header-bg); text-align: center; border-bottom: 1px solid var(--surface2);
+  --header-pad-y: 28px;
+  padding: var(--header-pad-y) 24px;
+  transition: padding 0.3s ease, box-shadow 0.3s ease;
+  flex-shrink: 0;
+}
+.header.compact {
+  --header-pad-y: 8px;
+  box-shadow: 0 2px 12px rgba(0,0,0,0.25);
+}
+.header h1 {
+  font-size: 26px; font-weight: 700; margin-bottom: 6px;
+  transition: font-size 0.3s ease, margin 0.3s ease;
+}
+.header.compact h1 { font-size: 18px; margin-bottom: 0; }
+.header p {
+  color: rgba(255,255,255,0.7); font-size: 14px;
+  transition: opacity 0.25s ease, max-height 0.3s ease, margin 0.3s ease;
+  max-height: 40px; overflow: hidden;
+}
+.header.compact p { opacity: 0; max-height: 0; margin: 0; }
+.theme-toggle {
+  position: absolute; top: 16px; right: 16px;
+  background: rgba(255,255,255,0.15); border: none; color: white;
+  width: 36px; height: 36px; border-radius: 8px; cursor: pointer;
+  font-size: 18px; transition: background 0.2s, top 0.3s ease;
+}
+.header.compact .theme-toggle { top: 8px; }
 .theme-toggle:hover { background: rgba(255,255,255,0.25); }
 
 /* Layout */
-.app { display: flex; height: calc(100vh - 100px); }
+.app { display: flex; flex: 1; min-height: 0; }
 .sidebar { width: 280px; background: var(--surface); border-right: 1px solid var(--surface2); padding: 16px; overflow-y: auto; flex-shrink: 0; }
 .main { flex: 1; padding: 24px; overflow-y: auto; }
 
@@ -1066,8 +1091,22 @@ function escapeHtml(s) {
   return s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
 }
 
+// ─── Scroll: compact header on scroll ───
+function setupHeaderScroll() {
+  const main = document.getElementById('mainContent');
+  const header = document.querySelector('.header');
+  if (!main || !header) return;
+
+  const THRESHOLD = 60;
+
+  main.addEventListener('scroll', () => {
+    header.classList.toggle('compact', main.scrollTop > THRESHOLD);
+  }, { passive: true });
+}
+
 // ─── Start ───
 init();
+setupHeaderScroll();
 </script>
 </body>
 </html>