feat: 实现 Content Script 原位轻量提示 UI

- 新增 uiPopover.ts 实现原位弹窗组件（深色主题、自动定位、自动隐藏）
- 新增 contextMenuHandler.ts 处理右键菜单消息
- 时间戳转换：在点击位置显示转换结果
- 文本统计：显示字符/单词/行数/字节统计
- 更新 messageHandler.ts 挂载右键菜单消息监听

entrypoints/content/contextMenuHandler.ts

@@ -0,0 +1,62 @@
+import { MessageAction, onMessage } from '@/utils/messages';
+import { getTextStats } from '@/utils/textStatistics';
+import { showTimestampResult, showTextStatsResult, hidePopover } from './uiPopover';
+import type { ContextMenuClickedPayload } from '@/utils/messages';
+
+function convertTimestamp(input: string): string {
+  const num = Number(input.trim());
+  if (isNaN(num)) {
+    return '无效时间戳';
+  }
+
+  const d = num > 1e12 ? new Date(num) : new Date(num * 1000);
+
+  if (isNaN(d.getTime())) {
+    return '无效时间戳';
+  }
+
+  const year = d.getFullYear();
+  const month = String(d.getMonth() + 1).padStart(2, '0');
+  const day = String(d.getDate()).padStart(2, '0');
+  const hours = String(d.getHours()).padStart(2, '0');
+  const minutes = String(d.getMinutes()).padStart(2, '0');
+  const seconds = String(d.getSeconds()).padStart(2, '0');
+
+  return `${year}/${month}/${day} ${hours}:${minutes}:${seconds}`;
+}
+
+let lastClickX = 0;
+let lastClickY = 0;
+
+document.addEventListener(
+  'contextmenu',
+  (e) => {
+    lastClickX = e.clientX;
+    lastClickY = e.clientY;
+  },
+  true,
+);
+
+export function initContextMenuHandler(): void {
+  onMessage(MessageAction.CONTEXT_MENU_CLICKED, (message) => {
+    const { featureKey, payload } = message.data as ContextMenuClickedPayload;
+
+    switch (featureKey) {
+      case 'timestamp': {
+        const result = convertTimestamp(payload);
+        showTimestampResult(lastClickX, lastClickY, payload, result);
+        break;
+      }
+
+      case 'textStatistics': {
+        const stats = getTextStats(payload);
+        showTextStatsResult(lastClickX, lastClickY, payload, stats);
+        break;
+      }
+
+      default:
+        hidePopover();
+        break;
+    }
+  });
+}