fix(site): proxy path-based workspace app routes in Vite dev server

During development, path-based workspace app URLs like
/@user/workspace.agent/apps/slug/ were not being proxied to the
backend. Vite's SPA fallback served index.html instead, requiring
developers to manually change the port to :3000.

Add proxy rules for /@ and /%40 prefixed paths that contain /apps/,
which are backend-handled workspace app routes. A bypass function
ensures other /@user/workspace paths continue to be served by Vite
as frontend routes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

site/vite.config.mts

@@ -81,6 +81,36 @@ export default defineConfig({
 				secure: process.env.NODE_ENV === "production",
 				rewrite: (path) => path.replace(/\/+/g, "/"),
 			},
+			// Path-based workspace apps are served by the backend. Without
+			// these rules, Vite serves index.html (SPA fallback) instead
+			// of proxying to the Go server.
+			"/@": {
+				changeOrigin: true,
+				target: process.env.CODER_HOST || "http://localhost:3000",
+				secure: process.env.NODE_ENV === "production",
+				ws: true,
+				bypass: (req) => {
+					// Only proxy requests that contain /apps/ in the path,
+					// which are workspace app routes handled by the backend.
+					// Other /@user/workspace paths are frontend routes.
+					if (req.url?.includes("/apps/")) {
+						return undefined; // Proxy the request.
+					}
+					return false; // Skip proxy, let Vite handle it.
+				},
+			},
+			"/%40": {
+				changeOrigin: true,
+				target: process.env.CODER_HOST || "http://localhost:3000",
+				secure: process.env.NODE_ENV === "production",
+				ws: true,
+				bypass: (req) => {
+					if (req.url?.includes("/apps/")) {
+						return undefined;
+					}
+					return false;
+				},
+			},
 			"/api": {
 				ws: true,
 				changeOrigin: true,