feat: convert server to TypeScript + add ERP API proxy

- Migrate server/index.js → modular TS structure (config, routes, services)
- Add ERP proxy: GET /api/erp/sales proxies Hono ERP API with server-side auth
- JWT token cached server-side, auto-refreshes on 401
- ERP credentials stay server-side only (no VITE_ prefix)
- Vite dev proxy routes /api/erp → localhost:3001
- Preserve existing Salla OAuth integration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/package.json

@@ -1,16 +1,23 @@
 {
-  "name": "hihala-salla-server",
+  "name": "hihala-server",
   "version": "1.0.0",
-  "description": "Backend server for Salla OAuth and API integration",
-  "main": "index.js",
+  "type": "module",
+  "description": "Backend server for ERP proxy and Salla integration",
+  "main": "src/index.ts",
   "scripts": {
-    "start": "node index.js",
-    "dev": "node --watch index.js"
+    "start": "tsx src/index.ts",
+    "dev": "tsx watch src/index.ts"
   },
   "dependencies": {
     "axios": "^1.6.0",
     "cors": "^2.8.5",
     "dotenv": "^16.3.1",
     "express": "^4.18.2"
+  },
+  "devDependencies": {
+    "@types/cors": "^2.8.17",
+    "@types/express": "^4.17.21",
+    "tsx": "^4.19.0",
+    "typescript": "^5.9.3"
   }
 }