chore: enhance data source configuration and mock data handling

- Added support for mock data mode in the frontend by introducing a new environment variable in the Dockerfile and workflow configuration.
- Updated the settings page to conditionally display options based on the availability of mock data, improving user experience.
- Refactored data source logic to include a check for mock data availability, ensuring proper mode handling in the application.

lib/backend-url.ts

@@ -13,9 +13,14 @@ export function configuredBackendUrl(): ConfiguredBackendUrl {
 }
 
 export function defaultDataSourceMode(): "mock" | "live" {
+  if (!isMockDataSourceAvailable()) return "live"
   return process.env.NEXT_PUBLIC_DEFAULT_DATA_SOURCE === "live" ? "live" : "mock"
 }
 
+export function isMockDataSourceAvailable(): boolean {
+  return process.env.NEXT_PUBLIC_ALLOW_MOCK_DATA !== "false"
+}
+
 export function isBackendUrlLocked(): boolean {
   const cfg = configuredBackendUrl()
   return cfg.kind === "same-origin" || cfg.kind === "fixed"