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/src/config.ts

@@ -0,0 +1,25 @@
+import dotenv from 'dotenv';
+import { fileURLToPath } from 'url';
+import { dirname, resolve } from 'path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+dotenv.config({ path: resolve(__dirname, '..', '.env') });
+
+export const server = {
+  port: parseInt(process.env.SERVER_PORT || '3001', 10),
+};
+
+export const salla = {
+  clientId: process.env.SALLA_CLIENT_ID || '',
+  clientSecret: process.env.SALLA_CLIENT_SECRET || '',
+  redirectUri: process.env.SALLA_REDIRECT_URI || 'http://localhost:3001/auth/callback',
+  accessToken: process.env.SALLA_ACCESS_TOKEN || '',
+  refreshToken: process.env.SALLA_REFRESH_TOKEN || '',
+};
+
+export const erp = {
+  apiUrl: process.env.ERP_API_URL || '',
+  apiCode: process.env.ERP_API_CODE || '',
+  username: process.env.ERP_USERNAME || '',
+  password: process.env.ERP_PASSWORD || '',
+};