feat(web): incremental render-window batch rendering engine

Add render_window.js: Python-driven incremental pagination with id+chainIndex state, anchor restore, batch guard and streaming auto-detect; wired through chat_bridge and the page shell.

ui/web/style.css

@@ -900,3 +900,33 @@ body, html {
 .message-wrapper.assistant .reply-content .md-segment + .md-segment {
     margin-top: 6px;
 }
+
+/* ==================== 🆕 P1-01 渲染窗口：加载入口按钮 ==================== */
+/* 上/下两端的「加载更多」入口：无边框浅灰胶囊，居中；无更多消息时由 JS 隐藏 */
+.load-window-btn {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    width: 220px;
+    margin: 14px auto;
+    padding: 6px 14px;
+    font-size: 12px;
+    color: #666;
+    background: #f4f4f6;
+    border: 1px solid #e3e3e8;
+    border-radius: 999px;
+    cursor: pointer;
+    user-select: none;
+    transition: background 0.15s ease, color 0.15s ease;
+}
+.load-window-btn:hover {
+    background: #eaeaf0;
+    color: #333;
+}
+.load-window-btn[hidden] {
+    display: none;
+}
+/* 底部入口与 scroll-anchor 的间距（scroll-anchor 自带 150px margin） */
+#load-newer {
+    margin-bottom: 4px;
+}