feat: dataset creation ui

[Scr4tch587]

What changed

New create view (`components/create-dataset.tsx`, reachable via the "new dataset" header button):

• Proposer section (required): name, Wat Street team, discord user, and a "what is this dataset for?" description — all shown to reviewers in the PR body
• Basics: name/version inputs, calendar + granularity selects, start-date
• Schema: dynamic key/type rows (str · int · float · bool)
• Dependencies: picker populated from `GET /datasets` plus optional lookback (`5d`-style, zod-validated)
• Builder script: CodeMirror editor with Python highlighting and a template pre-filled
• Extras: optional requirements.txt; env-vars checkbox reveals a `.env.template` field with a "secrets never go in the PR" note
• zod + react-hook-form validation mirrors the server's rules (name regex, semver, date format, lookback format), and server-side 400/409 messages render inline on submit
• success screen links the opened PR and repeats the manual-`.env` warning for env-vars datasets

The view is lazy-loaded (`React.lazy`) so CodeMirror's chunk (~540 kB) never loads during normal browsing. shadcn `select`/`checkbox`/`textarea` components added; new deps: `@uiw/react-codemirror`, `@codemirror/lang-python`.

Why

The UI half of the PR-based dataset-creation flow — pairs with the `POST /datasets` endpoint below this PR in the stack.

Benefit

Anyone on the team can propose a dataset without touching git; reviewers get a fully-attributed, pre-validated, pre-linted PR.

🤖 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 #385
                  • feat: dataset creation ui 👈
                  • docs: document dataset proposal flow in specs #387