feat: add API docs link button in sidebar footer

Opens Swagger UI (/docs) in a new tab. Proxies /docs and /openapi.json
in dev mode so the link works from the Vite dev server.

frontend/vite.config.ts

@@ -6,7 +6,9 @@ export default defineConfig({
 	plugins: [tailwindcss(), sveltekit()],
 	server: {
 		proxy: {
-			'/api': 'http://localhost:8420'
+			'/api': 'http://localhost:8420',
+			'/docs': 'http://localhost:8420',
+			'/openapi.json': 'http://localhost:8420'
 		}
 	}
 });