fix(chat): restore Facebook keyboard avoidance

src/app/chat/components/chat-input-bar.module.css

@@ -1,11 +1,14 @@
 /* ChatInputBar 输入栏容器样式（与 Dart chat_input_bar.dart 对齐） */
 
+/* Keyboard inset is a total bottom inset, not an addition to normal spacing. */
+
 .bar {
   flex: 0 0 auto;
   padding: 0
     calc(var(--chat-inline-padding, 16px) + var(--app-safe-right, 0px))
-    calc(
-      var(--spacing-lg, 16px) + var(--app-safe-bottom, 0px)
+    max(
+      calc(var(--spacing-lg, 16px) + var(--app-safe-bottom, 0px)),
+      var(--chat-keyboard-bottom-inset, 0px)
     )
     calc(var(--chat-inline-padding, 16px) + var(--app-safe-left, 0px));
   background: transparent;

src/app/chat/components/chat-input-bar.tsx

@@ -4,8 +4,8 @@ import { useRef, useState } from "react";
 import { useChatDispatch } from "@/stores/chat/chat-context";
 import { Logger } from "@/utils/logger";
 
+import { useChatKeyboardAvoidance } from "../hooks/use-chat-keyboard-avoidance";
 import { useChatKeyboardDiagnostics } from "../hooks/use-chat-keyboard-diagnostics";
-import { useChatVirtualKeyboardOverlayExperiment } from "../hooks/use-chat-virtual-keyboard-overlay-experiment";
 import { ChatInputTextField } from "./chat-input-text-field";
 import { ChatSendButton } from "./chat-send-button";
 import styles from "./chat-input-bar.module.css";
@@ -27,7 +27,10 @@ export function ChatInputBar({ disabled = false }: ChatInputBarProps) {
 
   const hasContent = input.trim().length > 0;
 
-  useChatVirtualKeyboardOverlayExperiment();
+  useChatKeyboardAvoidance({
+    active: isFocused,
+    containerRef: barRef,
+  });
   useChatKeyboardDiagnostics({ containerRef: barRef });
 
   const handleInputChange = (value: string) => {