feat: replace svelte frontend with react + tailwind scaffold

[Scr4tch587]

What changed

• removed the Svelte 5 app (components, svelte config, eslint/prettier svelte plugins, `bun.lockb`)
• scaffolded React 19 + TypeScript + Vite in its place, still managed with bun (`bun.lock` text lockfile)
• Tailwind CSS v4 (`@tailwindcss/vite`, CSS-first theme) + shadcn/ui (new-york style): generated `button input table dialog card form label sonner` into `src/components/ui/`
• ported the existing dark design tokens (`--background #0f1117`, `--primary #6c8cff`, etc.) into `src/index.css` as shadcn CSS variables — dark-only, no theme toggle, plus the `.json-*` highlight token colors for the upcoming JSON viewer
• `vite.config.ts`: keeps the `/api → localhost:3000` dev proxy; sets `base: "/datastream/"` in production builds for GitHub Pages
• vite now runs under the bun runtime (`bunx --bun vite`) so the installer and JS runtime always agree on native-binding architecture (fixes rollup/esbuild binding mismatches when the local node is a different arch than bun)
• eslint flat config (typescript-eslint + react-hooks + react-refresh); prettier with the tailwind class-sorting plugin
• minimal `App.tsx`: header shell only — API client, auth, and browsing UI land in the next PRs of this stack

Why

The frontend is being rebuilt in React for deployment to GitHub Pages, with a create-dataset UI planned later. This PR is the mechanical stack swap, kept separate so the feature PRs on top are reviewable.

Benefit

`package.json` keeps the exact same script names (`format:check`/`lint`/`typecheck`/`build`), so `.pre-commit-config.yaml` and `frontend-ci.yml` need zero changes — all existing hooks and CI pass unchanged.

🤖 Generated with Claude Code

[github-actions]

• main
  • feat: replace svelte frontend with react + tailwind scaffold 👈
    • feat: typed api client and api key handling #376
      • feat: dataset browsing ui #377
        • ci: deploy frontend to github pages #378
          • docs: update specs for react migration #379
            • feat: github rest client for dataset proposals #383
              • feat: dataset proposal service and endpoint #384
                • feat: api client support for dataset proposals #385
                  • feat: dataset creation ui #386
                  • docs: document dataset proposal flow in specs #387