fix: visual fixes — dark theme default, sidebar, dashboard cards, timeline dots, header counts

- Set data-theme=dark as default in index.html, useTheme defaults to 'dark' (issues 1, 10)
- Rename "Cron" to "Cron 管理" in sidebar (issue 2)
- Set card grid gap to 14px per design spec (issue 3)
- Show D-grade profile names below health bar chart (issue 4)
- Remove R/F/H/OK text from timeline, use colored dots with hover tooltip (issue 5)
- Wire live online/running/offline counts via usePolling in header, add offline badge (issue 9)
- Fix search placeholder to "搜索 Profile / 任务 / Cron / 事件..." (issue 9)

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

frontend/index.html

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="zh-CN">
+<html lang="zh-CN" data-theme="dark">
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />