fix(frontend): 多行输入时保持光标可见

adjustTextareaSize 重算高度会打乱 scrollTop，导致多行输入时光标被滚出视野。
新增 keepEditorSelectionVisible：高度重设后，聚焦态用编辑器 coordsAtPos 算出
光标位置并滚回可视区（上下留 lineHeight*0.3 padding），非聚焦态恢复原 scrollTop。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/src/components/input/InputComposer.vue

@@ -1528,6 +1528,9 @@ const adjustTextareaSize = () => {
     return;
   }
   const target = root;
+  const previousScrollTop = target.scrollTop;
+  const selectionShouldStayVisible =
+    document.activeElement === target || target.contains(document.activeElement);
   target.style.height = 'auto';
   const computedStyle = window.getComputedStyle(target);
   const lineHeight = parseFloat(computedStyle.lineHeight || '20') || 20;
@@ -1537,11 +1540,58 @@ const adjustTextareaSize = () => {
   const multiline = targetHeight > lineHeight * 1.4;
   applyLineMetrics(lines, multiline);
   target.style.height = `${targetHeight}px`;
+  keepEditorSelectionVisible(target, {
+    previousScrollTop,
+    selectionShouldStayVisible,
+    padding: Math.max(4, lineHeight * 0.3)
+  });
   nextTick(() => {
     emitComposerHeight();
   });
 };
 
+const keepEditorSelectionVisible = (
+  target: HTMLElement,
+  options: { previousScrollTop: number; selectionShouldStayVisible: boolean; padding: number }
+) => {
+  const maxScrollTop = Math.max(0, target.scrollHeight - target.clientHeight);
+  if (maxScrollTop <= 0) {
+    target.scrollTop = 0;
+    return;
+  }
+
+  if (!options.selectionShouldStayVisible) {
+    target.scrollTop = Math.min(maxScrollTop, Math.max(0, options.previousScrollTop));
+    return;
+  }
+
+  requestAnimationFrame(() => {
+    const instance = editor.value;
+    if (!instance) {
+      target.scrollTop = Math.min(maxScrollTop, Math.max(0, options.previousScrollTop));
+      return;
+    }
+
+    try {
+      const caretRect = instance.view.coordsAtPos(instance.state.selection.head);
+      const targetRect = target.getBoundingClientRect();
+      const padding = options.padding;
+      const currentMaxScrollTop = Math.max(0, target.scrollHeight - target.clientHeight);
+      let nextScrollTop = target.scrollTop;
+
+      if (caretRect.bottom > targetRect.bottom - padding) {
+        nextScrollTop += caretRect.bottom - targetRect.bottom + padding;
+      } else if (caretRect.top < targetRect.top + padding) {
+        nextScrollTop -= targetRect.top + padding - caretRect.top;
+      }
+
+      target.scrollTop = Math.min(currentMaxScrollTop, Math.max(0, nextScrollTop));
+    } catch {
+      target.scrollTop = Math.min(maxScrollTop, Math.max(0, options.previousScrollTop));
+    }
+  });
+};
+
 const onInput = (event: Event) => {
   const target = event.target as HTMLTextAreaElement;
   if (!props.isConnected || props.inputLocked) {