feat(desktop): Sidecar 健康度面板 + 重启后端能力

P1 已经把 backend-warning / backend-terminated 横幅做出来了；P2 把
lib.rs 那条 stdout/stderr/terminated 信息流真正落到一个常驻 UI 上：
- 右下角浮动状态点（绿/黄/红），轮询 /api/sys_health 决定颜色
- 点开抽屉看最近 200 行日志（ring buffer），含「重启后端」「复制日志」按钮

Rust：
- src-tauri/src/lib.rs：把 sidecar 启动抽出 spawn_backend_sidecar()，
  CommandChild 存进 SidecarHandle(Mutex<Option<CommandChild>>) 这个 state
- 新增 #[tauri::command] restart_backend_sidecar：kill 旧 child + 重新 spawn +
  emit 'backend-restarted' 给前端
- 监听任务 stdout/stderr emit 时不再用 format!("'{}'", ...) 包引号，原文直传；
  前端 hook 同时兼容旧形式（兜底剥引号）

前端：
- components/BackendHealth/useBackendEvents.ts：listen 四个事件 +
  ring buffer (MAX 200 行) + invoke restart + clipboard 复制日志
- BackendHealthIndicator.tsx：右下角浮动状态点，5s 轮询 /api/sys_health；
  连续 3 次失败或 backend-terminated 触发 → 红
- BackendLogPanel.tsx：右侧抽屉，深色 monospace 日志区 + 操作按钮
- 纯 web 环境（无 __TAURI_INTERNALS__）下静默不挂载

P3 / P4 还在路上。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

BillNote_frontend/src/components/BackendHealth/BackendHealthIndicator.tsx

@@ -0,0 +1,111 @@
+import { useEffect, useState } from 'react'
+import { useBackendEvents } from './useBackendEvents'
+import BackendLogPanel from './BackendLogPanel'
+
+// 健康度判定：
+// - 绿：sidecar running 且 /sys_health 通
+// - 黄：sidecar running 但 /sys_health 失败 (ffmpeg 缺等)
+// - 红：sidecar terminated 或 /sys_health 连续 3 次失败
+
+type Health = 'green' | 'yellow' | 'red' | 'unknown'
+
+const HEALTH_POLL_MS = 5000
+const SYS_HEALTH_PATH = '/api/sys_health'
+
+function backendBase(): string {
+  // 与 services/request.ts 用的一致
+  const fromEnv = (import.meta as any).env?.VITE_API_BASE_URL as string | undefined
+  return (fromEnv ?? '').replace(/\/$/, '')
+}
+
+const BackendHealthIndicator = () => {
+  const { status, isTauri, exitCode, logs, restart, copyLogs } = useBackendEvents()
+  const [open, setOpen] = useState(false)
+  const [healthCheckFailures, setHealthCheckFailures] = useState(0)
+  const [lastHealthOk, setLastHealthOk] = useState<boolean | null>(null)
+
+  // 仅在 Tauri 环境挂指示器；纯 web 用户由 useCheckBackend 接管
+  useEffect(() => {
+    if (!isTauri) return
+    let mounted = true
+
+    async function ping() {
+      try {
+        const res = await fetch(`${backendBase()}${SYS_HEALTH_PATH}`)
+        const ok = res.ok
+        if (!mounted) return
+        if (ok) {
+          setHealthCheckFailures(0)
+          setLastHealthOk(true)
+        }
+        else {
+          setHealthCheckFailures(c => c + 1)
+          setLastHealthOk(false)
+        }
+      }
+      catch {
+        if (!mounted) return
+        setHealthCheckFailures(c => c + 1)
+        setLastHealthOk(false)
+      }
+    }
+
+    ping()
+    const t = setInterval(ping, HEALTH_POLL_MS)
+    return () => {
+      mounted = false
+      clearInterval(t)
+    }
+  }, [isTauri])
+
+  if (!isTauri) return null
+
+  const health: Health = (() => {
+    if (status === 'terminated') return 'red'
+    if (healthCheckFailures >= 3) return 'red'
+    if (lastHealthOk === false) return 'yellow'
+    if (lastHealthOk === true) return 'green'
+    return 'unknown'
+  })()
+
+  const colorMap: Record<Health, string> = {
+    green: 'bg-green-500',
+    yellow: 'bg-amber-500',
+    red: 'bg-red-500',
+    unknown: 'bg-gray-400',
+  }
+
+  const labelMap: Record<Health, string> = {
+    green: '后端运行正常',
+    yellow: '后端运行中（部分检查未通过）',
+    red: status === 'terminated' ? `后端已退出 (code=${exitCode ?? 'unknown'})` : '后端无响应',
+    unknown: '后端状态未知',
+  }
+
+  return (
+    <>
+      <button
+        className="fixed right-3 bottom-3 z-[9998] flex items-center gap-2 rounded-full border bg-white px-3 py-1.5 text-xs shadow hover:shadow-md"
+        title={labelMap[health]}
+        onClick={() => setOpen(true)}
+      >
+        <span className={`inline-block h-2 w-2 rounded-full ${colorMap[health]}${health === 'red' || health === 'yellow' ? ' animate-pulse' : ''}`} />
+        <span className="text-gray-700">后端</span>
+      </button>
+
+      {open && (
+        <BackendLogPanel
+          status={status}
+          exitCode={exitCode}
+          logs={logs}
+          health={health}
+          onRestart={restart}
+          onCopyLogs={copyLogs}
+          onClose={() => setOpen(false)}
+        />
+      )}
+    </>
+  )
+}
+
+export default BackendHealthIndicator