feat(channel-monitor): add feature switch settings + fix extra_models save

Settings:
- New "功能开关" tab between 通用设置 and 安全与认证
- ChannelMonitorEnabled toggle: runner skips scheduling when false,
  user-facing list returns empty
- ChannelMonitorDefaultIntervalSeconds (15-3600): pre-fills interval
  when creating a new monitor; each monitor can still override

Bug fix:
- ModelTagInput now commits pending input on blur, not just Enter/Tab.
  Previously clicking "save" with an un-Enter'd extra model would drop
  the value (DB stored extra_models=[] even when user typed entries).

Backend:
- domain_constants: SettingKeyChannelMonitor{Enabled,DefaultIntervalSeconds}
- SettingService.GetChannelMonitorRuntime: lightweight getter used by
  runner tick + user handler per-request (fail-open on DB error)
- Runner tickDueChecks: bails early when feature disabled
- ChannelMonitorUserHandler: checks feature flag before serving
- Comment on runner doc: scheduler state is implicit (every tick re-reads
  ListEnabled from DB), so CRUD ops on monitors self-maintain the schedule

Bump VERSION to 0.1.114.25

backend/internal/service/settings_view.go

@@ -125,6 +125,10 @@ type SystemSettings struct {
 	OpsQueryModeDefault          string
 	OpsMetricsIntervalSeconds    int
 
+	// Channel Monitor feature
+	ChannelMonitorEnabled                 bool `json:"channel_monitor_enabled"`
+	ChannelMonitorDefaultIntervalSeconds  int  `json:"channel_monitor_default_interval_seconds"`
+
 	// Claude Code version check
 	MinClaudeCodeVersion string
 	MaxClaudeCodeVersion string
@@ -209,6 +213,10 @@ type PublicSettings struct {
 	AccountQuotaNotifyEnabled   bool
 	BalanceLowNotifyThreshold   float64
 	BalanceLowNotifyRechargeURL string
+
+	// Channel Monitor feature
+	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
+	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
 }
 
 type WeChatConnectOAuthConfig struct {