feat: api client support for dataset proposals

[Scr4tch587]

What changed

• `lib/api.ts`: `apiFetch` gains an options object (method, JSON body, ok-statuses) and now extracts the backend's `detail` message from error bodies, so `ApiError.message` carries the server's actual validation error instead of a generic status line
• new `proposeDataset()` + typed `DatasetProposalPayload` / `ProposalResponse` for `POST /api/v1/datasets`
• `main.tsx`: the QueryClient's 401 handling (toast + reopen key dialog) now also covers mutations via a `MutationCache`, sharing one `handleAuthError`

Why

The create-dataset form (next PR) submits proposals and must surface the backend's precise 400/409 messages ("granularity is finer than dependency ...") inline in the form.

Benefit

Client layer for the proposal flow with no UI yet — small, independently reviewable. Existing fetch call sites are updated to the new signature with identical behavior.

🤖 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 #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 👈
                  • feat: dataset creation ui #386
                  • docs: document dataset proposal flow in specs #387
                  • feat: api client support for triggering builds #388
                  • feat: build trigger ui #389
                  • docs: document build trigger in frontend spec #390