fix(gateway): debounce restart with single-flight queue (#248)

Root cause for #243 / #244 / #240: model edits trigger
api.restartGateway() with only 300ms debounce. Fast consecutive
edits stack up restart calls, creating zombie Gateway processes,
failed restarts, and CPU fan spikes.

Layer A (frontend):
- New src/lib/gateway-restart-queue.js: 3s debounce + single-flight
  lock + reschedule on in-flight request
- Refactor src/pages/models.js doAutoSave: write config immediately,
  schedule restart via queue with 'Apply now' toast button
- Subscribe to queue state for unified success/failure toast
- Add i18n: models.configQueued, models.applyNow

Layer B (backend):
- src-tauri/src/commands/config.rs: wrap restart_gateway /
  reload_gateway with tokio::sync::Mutex + 2s cooldown
- Cargo.toml: add tokio 'sync' feature
- scripts/dev-api.js: same guard for Web mode (inflight promise
  reuse + 2s cooldown)

Effects:
- 10 rapid edits within 3s -> 1 restart (was 10+ with races)
- Backend serializes concurrent restart calls, no zombie spawns
- User sees single 'Apply now' toast instead of restart storm

Refs #243 #244 #240

src/lib/gateway-restart-queue.js

@@ -0,0 +1,153 @@
+/**
+ * Gateway 重启防抖队列
+ *
+ * 解决 issue #243 #244 #240：用户快速连续改动配置时，之前会触发多次 Gateway 重启
+ * 导致僵尸进程堆积、风扇狂转、重启失败等问题。
+ *
+ * 设计：
+ *   - 默认 3s 空闲防抖：连续编辑时每次重置计时
+ *   - 单飞行锁：前一次重启未完成时，新请求只设"补重启"标记
+ *   - 立即执行入口：UI 提供 "立即重载" 按钮跳过倒计时
+ *   - 事件订阅：供顶部状态条 / toast 展示倒计时与结果
+ *
+ * 对外 API：
+ *   scheduleGatewayRestart({ delay, reason })  // 入队
+ *   fireRestartNow()                            // 跳过倒计时
+ *   cancelPendingRestart()                      // 取消
+ *   hasPendingRestart() / isRestartInFlight()   // 状态查询
+ *   onRestartState(cb)                          // 订阅状态变化
+ */
+
+import { api } from './tauri-api.js'
+
+const DEFAULT_DELAY_MS = 3000
+const RESCHEDULE_DELAY_MS = 500
+
+let _pendingTimer = null
+let _scheduledAt = 0
+let _scheduledDelay = 0
+let _currentReason = ''
+let _inflight = false
+let _needRerun = false
+let _listeners = []
+
+function emit(eventName, detail = {}) {
+  const payload = {
+    event: eventName,
+    reason: _currentReason,
+    pending: hasPendingRestart(),
+    inflight: _inflight,
+    scheduledAt: _scheduledAt,
+    delay: _scheduledDelay,
+    ...detail,
+  }
+  _listeners.forEach(fn => {
+    try { fn(payload) } catch (_) { /* 忽略订阅方异常 */ }
+  })
+}
+
+/**
+ * 预约一次 Gateway 重启。多次调用会合并为一次。
+ * @param {Object} opts
+ * @param {number} [opts.delay=3000] 空闲多久后触发（毫秒）
+ * @param {string} [opts.reason='config-change'] 触发原因（用于日志/UI）
+ */
+export function scheduleGatewayRestart(opts = {}) {
+  const delay = Number.isFinite(opts.delay) ? opts.delay : DEFAULT_DELAY_MS
+  const reason = opts.reason || 'config-change'
+
+  if (_pendingTimer) clearTimeout(_pendingTimer)
+  _scheduledAt = Date.now()
+  _scheduledDelay = delay
+  _currentReason = reason
+
+  if (_inflight) {
+    _needRerun = true
+    emit('deferred')
+    return
+  }
+
+  _pendingTimer = setTimeout(runRestart, delay)
+  emit('scheduled')
+}
+
+/**
+ * 跳过倒计时，立即执行重启。
+ */
+export function fireRestartNow() {
+  if (_pendingTimer) {
+    clearTimeout(_pendingTimer)
+    _pendingTimer = null
+  }
+  if (_inflight) {
+    _needRerun = true
+    emit('deferred')
+    return
+  }
+  runRestart()
+}
+
+/**
+ * 取消待执行的重启。用户显式拒绝、页面卸载时调用。
+ */
+export function cancelPendingRestart() {
+  if (_pendingTimer) {
+    clearTimeout(_pendingTimer)
+    _pendingTimer = null
+  }
+  _needRerun = false
+  _scheduledAt = 0
+  emit('cancelled')
+}
+
+export function hasPendingRestart() {
+  return _pendingTimer !== null
+}
+
+export function isRestartInFlight() {
+  return _inflight
+}
+
+export function getPendingInfo() {
+  if (!_pendingTimer) return null
+  const elapsed = Date.now() - _scheduledAt
+  return {
+    reason: _currentReason,
+    delay: _scheduledDelay,
+    remaining: Math.max(0, _scheduledDelay - elapsed),
+  }
+}
+
+/**
+ * 订阅重启状态事件。返回取消订阅函数。
+ * 事件类型：
+ *   - scheduled / deferred / cancelled
+ *   - started / succeeded / failed
+ */
+export function onRestartState(fn) {
+  _listeners.push(fn)
+  return () => {
+    _listeners = _listeners.filter(cb => cb !== fn)
+  }
+}
+
+async function runRestart() {
+  _pendingTimer = null
+  _inflight = true
+  emit('started')
+
+  try {
+    const result = await api.restartGateway()
+    emit('succeeded', { result })
+  } catch (err) {
+    emit('failed', { error: err?.message ? err.message : String(err) })
+  } finally {
+    _inflight = false
+  }
+
+  // 运行期间有新请求 → 稍等 500ms 再跑一次
+  if (_needRerun) {
+    _needRerun = false
+    scheduleGatewayRestart({ delay: RESCHEDULE_DELAY_MS, reason: 'rescheduled' })
+  }
+}