Fix a few explorer frontend bugs (#4849)

## Motivation

The explorer frontend hardcodes API URLs to `http://localhost:3002`,
which only works in local development when both frontend and backend run
on localhost. This breaks in production deployments where:

- Frontend is served through Kubernetes ingress
- Backend API is accessed via the same ingress with `/api` path prefix
- Browsers block mixed content (HTTPS frontend → HTTP localhost backend)
- CORS issues arise from cross-origin requests

The hardcoded URLs prevent the explorer from working in deployed
environments.

## Proposal

Replace hardcoded absolute URLs with relative paths that work in all
environments. Add Vite dev server proxy configuration to maintain local
development workflow while using relative paths.

This approach works in all environments:
- **Local dev**: Vite proxy forwards `/api` → `http://localhost:3002`
- **Production**: Kubernetes ingress routes `/api` → backend service

## Test Plan

1. Test production deployment:
   - Deploy explorer with Kubernetes ingress configured
   - Verify frontend loads and connects to backend
   - Confirm no CORS or mixed content errors
   - Verify all API calls work through ingress

## Release Plan

- These changes should be backported to the latest `testnet` branch,
then
    - be released in a validator hotfix.

Author: ndr-ds

linera-explorer-new/src/hooks/useDatabase.ts

@@ -9,10 +9,10 @@ export const useAPI = () => {
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
-    // Test API connection
+    // Test API connection using relative path
     const testConnection = async () => {
       try {
-        const response = await fetch('http://localhost:3002/api/health');
+        const response = await fetch('/api/health');
         if (response.ok) {
           setIsConnected(true);
         } else {

linera-explorer-new/src/utils/database.ts

@@ -1,6 +1,7 @@
 import { Block, BlockInfo, IncomingBundle, PostedMessage, ChainInfo, Operation, Message, Event, OracleResponse, IncomingBundleWithMessages } from '../types/blockchain';
 
-const API_BASE_URL = 'http://localhost:3002/api';
+// Use relative path for API calls - works in all environments (local dev via proxy, production via ingress)
+const API_BASE_URL = '/api';
 
 export class BlockchainAPI {
   // Get all blocks with pagination

linera-explorer-new/vite.config.ts

@@ -4,6 +4,12 @@ import react from '@vitejs/plugin-react'
 export default defineConfig({
   plugins: [react()],
   server: {
-    port: 3001
+    port: 3001,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:3002',
+        changeOrigin: true
+      }
+    }
   }
 })