feat: Enhance frontend with theme support and offline capabilities

### Summary of Changes
- Introduced theme-aware CSS variables for consistent styling across light and dark modes.
- Updated `App.jsx` to manage theme settings and improve layout responsiveness.
- Refactored `OfflineMode` component to provide detailed connection status and quick actions for users.
- Enhanced `Dashboard`, `Settings`, and `SystemMetrics` components to utilize new theme variables and improve UI consistency.
- Updated service URLs in constants and API documentation to reflect new configurations.

### Expected Results
- Improved user experience with a cohesive design that adapts to user preferences.
- Enhanced offline functionality, providing users with better feedback and control during service outages.
- Streamlined codebase with consistent styling practices, making future updates easier.

frontend/rsbuild.config.js

@@ -34,6 +34,11 @@ export default defineConfig({
     entry: {
       index: './src/index.js',
     },
+    define: {
+      'process.env.REACT_APP_API_URL': JSON.stringify(process.env.REACT_APP_API_URL || 'http://localhost:8080'),
+      'process.env.REACT_APP_ADAPTERS_URL': JSON.stringify(process.env.REACT_APP_ADAPTERS_URL || 'http://localhost:8000'),
+      'process.env.REACT_APP_DOCS_URL': JSON.stringify(process.env.REACT_APP_DOCS_URL || 'http://localhost:8083'),
+    },
   },
   tools: {
     rspack: {