[produckq-boop]

Problem

On Vercel, the static deployment has no local /api/ proxy like Vite's dev server or Docker's nginx.conf. In addition, several callers (BackendStatusBanner, ExportButton, SimulateButton, useEventStream, useTxStatus, and sandbox-api) bypassed resolveApiBase() in src/api.ts with hardcoded /api/... paths or window.location.host WebSocket URLs, so setting VITE_API_URL on Vercel still left health checks, SSE streams, exports, simulations, and sandbox endpoints hitting the static host and failing.

Changes

• vercel.json: Tighten the SPA rewrite regex to /((?!api(?:/|$)).*) -> /index.html so both /api and /api/* are excluded from the HTML fallback when unconfigured.
• src/api.ts: Normalize trailing slashes and /api suffixes in resolveApiBase(), and add buildApiUrl() and resolveWsUrl() so REST, SSE, and WebSocket callers resolve against VITE_API_URL when set and fall back to relative /api (or window.location) in dev and Docker/nginx.
• Call-site alignment: Route BackendStatusBanner, ExportButton, SimulateButton, useEventStream, useTxStatus, and sandbox-api through buildApiUrl() / resolveWsUrl(), and reject text/html fallback responses in BackendStatusBanner.checkHealth().
• Docs & env template: Document all three deployment targets (Vite dev proxy, Docker/nginx, and Vercel via VITE_API_URL or /api/:path* rewrite) in README.md and align .env.example.
• Tests: Expand test/api-base.test.ts and test/BackendStatusBanner.test.tsx to cover resolveApiBase, buildApiUrl, resolveWsUrl, sandbox-api paths, VITE_API_URL health checks, and HTML fallback rejection.

Testing

• npm run lint (0 errors)
• npx tsc --noEmit (clean)
• npm run build (passes)
• npm test (20 test files, 174 tests passed)

Closes #21

[vercel]

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