Add proxy configuration for single-port development

Configure SvelteKit dev server to proxy API requests to Elysia backend,
allowing users to access the application on a single port (5173).

## Changes
- Add proxy config to vite.config.js for /api requests
- Update API client to use relative URLs (/api)
- Add convenience 'dev' script to start both servers
- Update README with single-port instructions
- Add architecture section explaining proxy setup

Benefits:
- Single port to access (5173)
- HMR still works for frontend
- No CORS issues
- Backend runs hidden on port 3001

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -104,28 +104,25 @@ This creates the SQLite database with required tables (users, qsos, sync_jobs).
 
 ## Running the Application
 
-Start both backend and frontend:
+Start both backend and frontend with a single command:
 
 ```bash
-# Start backend (port 3001)
-bun run backend/index.js
-
-# Start frontend (port 5173)
-cd src/frontend && bun run dev
+bun run dev
 ```
 
-Or use the convenience scripts:
+Or start them individually:
 ```bash
-# Backend only
-bun run backend
+# Backend only (port 3001, proxied)
+bun run dev:backend
 
-# Frontend only
-bun run frontend
+# Frontend only (port 5173)
+bun run dev:frontend
 ```
 
 The application will be available at:
-- Frontend: http://localhost:5173
-- Backend API: http://localhost:3001
+- **Frontend & API**: http://localhost:5173
+
+**Note**: During development, both servers run (frontend on 5173, backend on 3001), but API requests are automatically proxied through the frontend. You only need to access port 5173.
 
 ## API Endpoints
 
@@ -201,6 +198,25 @@ CREATE TABLE sync_jobs (
 );
 ```
 
+## Architecture
+
+### Development Mode
+- **SvelteKit Dev Server** (port 5173): Serves frontend and proxies API requests
+- **Elysia Backend** (port 3001): Handles API requests (hidden from user)
+- **Proxy Configuration**: All `/api/*` requests are forwarded from SvelteKit to Elysia
+
+This gives you:
+- ✅ Single port to access (5173)
+- ✅ Hot Module Replacement (HMR) for frontend
+- ✅ No CORS issues
+- ✅ Simple production deployment
+
+### Production Mode
+In production, you can either:
+1. **Build static files** and serve from Elysia
+2. **Keep the proxy setup** with a proper reverse proxy (nginx, caddy)
+3. **Use SvelteKit adapter** for Node/Bun to serve everything from one process
+
 ## Features in Detail
 
 ### Background Job Queue