feat: implement single binary build and env-based auth

- Unify login page styling with share page
- Add AGENTS.md with build commands and code style guidelines
- Add dev.sh and make.sh for development and production builds
- Implement single binary build with embedded frontend using embed.FS
- Change auth configuration from CLI flag to env variables (USHARE_USERNAME, USHARE_PASSWORD)
- Set default credentials: admin / ushare@123
- Fix static file serving for SPA routes

frontend/vite.config.ts

@@ -5,6 +5,7 @@ import react from '@vitejs/plugin-react'
 export default defineConfig({
   plugins: [react()],
   server: {
+    host: '0.0.0.0',
     proxy: {
       '/api': {
         target: 'http://127.0.0.1:9119',