feat(channel-monitor): gate UI by feature switch + polish form UX

- AppSidebar 三处菜单项（管理端渠道监控、用户端/个人页渠道状态）按
  channel_monitor_enabled 条件展开，关闭时隐藏
- ChannelStatusView setInterval 随开关启停：关闭 clearInterval，
  开启/未知态自动启动，避免禁用功能后仍在轮询
- MonitorFormDialog provider Select 改为 3 色单选按钮
  （openai=emerald / anthropic=orange / gemini=sky），i18n 文案
  供应商 → 平台 / Provider → Platform
- MonitorKeyPickerDialog 按钮列表改为 name/key/group 三列表格 +
  搜索框，按 key.group.platform === provider 过滤，避免跨平台误选
- form.provider 变化时清空 api_key，修复切换平台仍保留旧 key 的
  错配 bug
- providerPickerClass 抽取到 useChannelMonitorFormat composable，
  统一 emerald/orange/sky 颜色语义，消除硬编码 Tailwind class 重复
- maskApiKey 工具函数统一（utils/maskApiKey.ts），KeysView 与
  MonitorKeyPickerDialog 共用 slice(0,6)...slice(-4) 策略
- bump version to 0.1.114.27

frontend/src/views/user/ChannelStatusView.vue

@@ -160,14 +160,34 @@ watch(items, () => {
   void ensureDetailsForWindow()
 })
 
+function startTimer() {
+  if (countdownTimer !== undefined) return
+  countdownTimer = setInterval(tick, 1000) as unknown as number
+}
+
+function stopTimer() {
+  if (countdownTimer !== undefined) {
+    clearInterval(countdownTimer)
+    countdownTimer = undefined
+  }
+}
+
+watch(
+  () => appStore.cachedPublicSettings?.channel_monitor_enabled,
+  (enabled) => {
+    if (enabled === false) stopTimer()
+    else startTimer()
+  },
+)
+
 // ── Lifecycle ──
 onMounted(() => {
   void reload(false)
-  countdownTimer = setInterval(tick, 1000) as unknown as number
+  if (appStore.cachedPublicSettings?.channel_monitor_enabled !== false) startTimer()
 })
 
 onBeforeUnmount(() => {
-  if (countdownTimer !== undefined) clearInterval(countdownTimer)
+  stopTimer()
   if (abortController) abortController.abort()
 })
 </script>