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/data-source.tsx

@@ -5,6 +5,7 @@ import {
   configuredBackendUrl,
   defaultDataSourceMode,
   isBackendUrlLocked,
+  isMockDataSourceAvailable,
   LOCAL_DEFAULT_BACKEND_URL,
   resolveStoredBackendUrl,
 } from "@/lib/backend-url"
@@ -19,6 +20,7 @@ interface DataSourceContextValue {
   backendUrl:    string
   setBackendUrl: (url: string) => void
   backendUrlLocked: boolean
+  mockModeAvailable: boolean
   /** undefined = не проверялось, true = OK, false = недоступен */
   backendStatus: boolean | undefined
   checkBackend:  () => Promise<void>
@@ -32,9 +34,12 @@ const LS_MODE    = "routerlists:data-source"
 const LS_BACKEND = "routerlists:backend-url"
 
 function readStoredMode(): DataSourceMode {
+  if (!isMockDataSourceAvailable()) return "live"
   if (typeof window === "undefined") return defaultDataSourceMode()
   const stored = localStorage.getItem(LS_MODE) as DataSourceMode | null
-  return stored === "mock" || stored === "live" ? stored : defaultDataSourceMode()
+  if (stored === "live") return "live"
+  if (stored === "mock") return "mock"
+  return defaultDataSourceMode()
 }
 
 function readStoredBackendUrl(): string {
@@ -55,6 +60,7 @@ export function DataSourceProvider({ children }: { children: React.ReactNode })
   )
   const [backendStatus, setBackendStatus] = useState<boolean | undefined>(undefined)
   const backendUrlLocked = isBackendUrlLocked()
+  const mockModeAvailable = isMockDataSourceAvailable()
 
   useEffect(() => {
     if (configuredBackendUrl().kind !== "same-origin") return
@@ -62,9 +68,10 @@ export function DataSourceProvider({ children }: { children: React.ReactNode })
   }, [])
 
   const setMode = useCallback((m: DataSourceMode) => {
+    if (!mockModeAvailable && m === "mock") return
     setModeState(m)
     localStorage.setItem(LS_MODE, m)
-  }, [])
+  }, [mockModeAvailable])
 
   const setBackendUrl = useCallback((url: string) => {
     if (backendUrlLocked) return
@@ -89,7 +96,7 @@ export function DataSourceProvider({ children }: { children: React.ReactNode })
 
   return (
     <DataSourceContext.Provider
-      value={{ mode, setMode, backendUrl, setBackendUrl, backendUrlLocked, backendStatus, checkBackend }}
+      value={{ mode, setMode, backendUrl, setBackendUrl, backendUrlLocked, mockModeAvailable, backendStatus, checkBackend }}
     >
       {children}
     </DataSourceContext.Provider>