chore(frontend): let vite preview reach the API

The proxy was only declared under `server`, so `vite preview` served the
production build with no route to the backend: every /api/v1 call 404'd and
the built app could not get past the login screen.

That left the production bundle effectively untestable, which is where
chunking — and the bug that made every route download recharts — only ever
shows up; `vite dev` does not chunk at all.

Reuse the dev proxy for preview, on VITE_PORT+100 so it cannot collide with
the dev server. Verified: preview serves on 8100 and /api/v1/info returns 200
through the proxy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -138,6 +138,9 @@ export default defineConfig(({ mode }) => {
                 '@': path.resolve(__dirname, './src'),
             },
         },
+        // `vite preview` serves the production build, which is the only place chunking exists —
+        // but it has no proxy of its own, so without this the built app cannot reach the API.
+        preview: { ...serverConfig, port: vitePort + 100 },
         server: serverConfig,
     };
 });