fix(frontend): add available_channels_enabled to PublicSettings type and defaults

featureFlags.ts registry uses 'available_channels_enabled' as a
public-settings key, but the PublicSettings TS type (types/index.ts)
and the app store default (stores/app.ts) only had
channel_monitor_enabled. Adds the missing field so pnpm build passes.

frontend/src/stores/app.ts

@@ -354,6 +354,7 @@ export const useAppStore = defineStore('app', () => {
         balance_low_notify_threshold: 0,
         channel_monitor_enabled: true,
         channel_monitor_default_interval_seconds: 60,
+        available_channels_enabled: false,
       }
     }
 

frontend/src/types/index.ts

@@ -187,6 +187,7 @@ export interface PublicSettings {
   balance_low_notify_threshold: number
   channel_monitor_enabled: boolean
   channel_monitor_default_interval_seconds: number
+  available_channels_enabled: boolean
 }
 
 export interface AuthResponse {