ci: deploy frontend to github pages

[Scr4tch587]

What changed

New `.github/workflows/deploy-pages.yml`: on push to `main` touching `frontend/**` (or manual dispatch), builds the frontend with bun and deploys `frontend/dist` to GitHub Pages via `actions/deploy-pages`. The production build already uses `base: "/datastream/"` (set in the scaffold PR), so assets resolve under `https://wat-street.github.io/datastream/\`.

One-time manual setup (repo admins)

1. Settings → Pages → Source = "GitHub Actions"
2. Settings → Secrets and variables → Actions → Variables: add `VITE_API_BASE_URL` = the deployed API origin (the Caddy `DOMAIN`, e.g. `https://api.example.com\` — no trailing slash, no `/api/v1`). It's baked into the bundle at build time (verified: the URL appears in `dist/assets` when set). If unset, the app falls back to relative `/api`, which won't work on Pages.

Note: Pages serves over https, so the API origin must be https too (browsers block mixed content; localhost is exempt for local testing).

Why

Gets the React app publicly hosted with zero server maintenance — the API stays wherever Caddy fronts it, with CORS (PR #374) allowing the Pages origin.

🤖 Generated with Claude Code

[github-actions]

• main
  • feat: replace svelte frontend with react + tailwind scaffold #375
    • feat: typed api client and api key handling #376
      • feat: dataset browsing ui #377
        • ci: deploy frontend to github pages 👈
          • docs: update specs for react migration #379