Add guided choice poster builder - #86

Motivation

• Add a simple guided mode so users can create shareable choice posters from one multiple-choice question and surface it under the /build/simple route using the BetterPosters-style centered card over a scrolling poster grid.
• Provide a lightweight, client-hydrated React UI that can be used as an entrypoint for future guided poster flows and quick poster generation UX.

Description

• Added a new React component SimpleChoiceBuilder at src/components/builder/SimpleChoiceBuilder.tsx that renders an editable question, add/remove choice controls, selected-choice toggles, generated SVG poster-tile backdrop (data-URI), and a live preview of the resulting poster choice.
• Created the new Astro route src/pages/build/simple.astro that mounts the component with client:only="react", includes page SEO metadata, and exposes the page at /build/simple.
• Appended UI styling to src/styles/global.css to implement the dark full-screen layout, scrolling poster grid background, radial overlay, centered glass card, responsive rules, and preview styles to match the requested layout.
• Small accessibility and robustness touches such as aria labels on the builder and preview, and an empty jsonLd array in the page frontmatter to avoid build-time schema errors.

Testing

• Ran npm run build and the static site build completed successfully (built /build/simple.html).
• Attempted npm run typecheck but TypeScript checks are blocked by pre-existing, unrelated type errors elsewhere in the repo (observed failures in admin/dashboard-related files), so a full typecheck of the new files could not be validated end-to-end.
• Attempted an automated visual capture with Playwright, but the environment lacks required system libraries and failed with a missing libatk-1.0.so.0, preventing headless Chromium screenshot validation.

Codex Task