fix(settings): inject channel_monitor & available_channels into SSR payload

Root cause: GetPublicSettingsForInjection used an inline struct that silently
drifted from dto.PublicSettings and omitted channel_monitor_enabled /
available_channels_enabled. On refresh window.__APP_CONFIG__ lacked these
keys, so cachedPublicSettings.available_channels_enabled resolved to
undefined and the opt-in sidebar entry (=== true) disappeared.

Backend: extract PublicSettingsInjectionPayload as a named type with all
feature-flag fields wired, and add a reflect-based drift test in the dto
package so forgetting a future flag fails CI instead of the browser.

Frontend: introduce utils/featureFlags.ts as the single registry for
public-settings-driven toggles, with explicit opt-in / opt-out modes that
encode the pre-load fallback. AppSidebar switches to makeSidebarFlag() so
adding a new switch only touches the registry.

backend/internal/service/setting_service.go

@@ -675,6 +675,8 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		AccountQuotaNotifyEnabled        bool            `json:"account_quota_notify_enabled"`
 		BalanceLowNotifyThreshold        float64         `json:"balance_low_notify_threshold"`
 		BalanceLowNotifyRechargeURL      string          `json:"balance_low_notify_recharge_url"`
+		ChannelMonitorEnabled            bool            `json:"channel_monitor_enabled"`
+		AvailableChannelsEnabled         bool            `json:"available_channels_enabled"`
 	}{
 		RegistrationEnabled:              settings.RegistrationEnabled,
 		EmailVerifyEnabled:               settings.EmailVerifyEnabled,
@@ -713,6 +715,8 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		AccountQuotaNotifyEnabled:        settings.AccountQuotaNotifyEnabled,
 		BalanceLowNotifyThreshold:        settings.BalanceLowNotifyThreshold,
 		BalanceLowNotifyRechargeURL:      settings.BalanceLowNotifyRechargeURL,
+		ChannelMonitorEnabled:            settings.ChannelMonitorEnabled,
+		AvailableChannelsEnabled:         settings.AvailableChannelsEnabled,
 	}, nil
 }