docs: add a screenshot banner to the README

[NeoVand]

What changed

The README opened with the logo and a one-line pitch, so nothing showed what Voicebook actually looks like until you clicked through to the live app. This adds the onboarding screen — "Choose how Voicebook reads", with the on-device / bring-your-own-key split for both the reading voice and visual descriptions — as a banner directly under the badges, linked to the live app.

The image was dropped in the repo root as screenshot.png; it now lives at docs/images/voicebook-onboarding.png, a new folder alongside the other docs, so it travels with the documentation instead of the deployed static/ bundle. Alt text describes the choice the screen presents rather than naming the file.

Docs only — no source, storage, or network behavior touched.

Verification

• npm run lint (prettier + eslint clean)
• npm run check — not run; no TypeScript or Svelte files changed
• npm run test:coverage — not run; no application code changed
• Relevant Chromium/visual journey — not applicable

Review checklist

• Privacy and network behavior are unchanged or explicitly explained. — unchanged; the banner is a repo-relative image, no third-party embed.
• Storage/migration and model-license implications are covered. — none.
• Keyboard, screen-reader, contrast, and reduced-motion behavior are covered. — the image carries descriptive alt text; nothing in the app changed.
• Visual changes include desktop screenshots. — the change is a desktop screenshot; render it on the PR's Files tab to check it.

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