docs: add Playwright screenshot suite and quick user guide (#594)

[grzanka]

First increment toward #594 (automate documentation screenshots with Playwright). Adds the local screenshot tooling, two primary-tool pages captured at desktop + mobile, and a short end-user guide that embeds them.

What's in this PR

• tests/docs-screenshots.spec.ts — a dedicated screenshot suite capturing the Calculator and Plot pages at desktop (1280×720) and mobile (375×667). Kept outside tests/e2e/ so it never runs in the normal E2E pipeline. Freezes all animations/transitions/caret and masks the dynamic build-info footer (neutral grey) for deterministic, churn-free PNGs.
• playwright.docs.config.ts + pnpm docs:screenshots — a separate config so the suite is excluded from CI E2E but runnable on demand.
  • Why a separate config rather than the playwright test <file> form suggested in the issue: the default playwright.config.ts restricts discovery to tests/e2e/ (testDir), so an explicit path outside that dir finds no tests.
• docs/assets/ — four generated screenshots (calculator/plot × desktop/mobile) showing live data (proton in liquid water at 100 MeV → 0.7286 keV/µm, CSDA range 7.721 cm; real stopping-power curve).
• docs/user-guide.md — a concise two-minute Calculator + Plot tour embedding the screenshots, linked from docs/README.md.
• Index/log updates: docs/README.md, CHANGELOG-AI.md, docs/ai-logs/.

Notes

• The screenshots need the WASM engine to render real data. WASM is gitignored, so it is not committed here — the images are regenerated wherever the binaries exist (CI, or a local build via wasm/build.sh).
• Verified locally: all 4 screenshot tests pass, pnpm format:check clean, ESLint clean on the new files.

Deferred to a follow-up

The GitHub Actions workflow_dispatch job that runs the script and auto-commits refreshed images, plus the "screenshots out of sync" PR check. This local tooling is the prerequisite for both.

https://claude.ai/code/session_01VfY2vhwkrs3vzBaDwzpx9x

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Generated by Claude Code

[Copilot]

Pull request overview

Adds a dedicated Playwright-driven documentation screenshot suite (kept out of the normal E2E pipeline) plus a short end-user guide that embeds the generated images, along with the required doc indexes / AI logs.

Changes:

• Introduces a standalone Playwright config + test suite to generate deterministic Calculator/Plot screenshots for docs (pnpm docs:screenshots).
• Adds a quick user guide page and links it from the docs index.
• Records the session in CHANGELOG-AI.md and docs/ai-logs/.

Reviewed changes

Copilot reviewed 8 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/docs-screenshots.spec.ts	New Playwright spec that navigates to Calculator/Plot and writes deterministic PNGs to docs/assets/.
playwright.docs.config.ts	New Playwright config to run only the docs screenshot suite via a dedicated invocation.
package.json	Adds docs:screenshots script that runs Playwright with the new config.
docs/user-guide.md	New end-user “quick tour” doc embedding the screenshots.
docs/README.md	Adds the user guide to the docs index table.
docs/ai-logs/README.md	Adds the new session log to the AI logs index.
docs/ai-logs/2026-06-05-issue-594-screenshots-guide.md	New detailed AI session log for the work done toward issue #594.
CHANGELOG-AI.md	Adds a changelog entry for the session (newest-first).