fix: BROWSE validation message was clipped away; refresh docs + screenshots

[DDecoene]

The bug

The BROWSE per-cell validation message shipped in #45 was never visible to anyone.

#grid-table td sets overflow: hidden (to ellipsis long values), which clipped the absolutely-positioned .cell-error tooltip to nothing. An invalid edit showed a red border and no reason at all.

Why the tests didn't catch it

toContainText() and toBeVisible() do not account for clipping by an ancestor's overflow. The element was in the DOM, had display: block; visibility: visible, and a real bounding box — Playwright happily reported isVisible: true. It was simply painted nowhere.

The assertion now uses toBeInViewport(), which is backed by an IntersectionObserver and therefore does account for ancestor clipping. I verified it fails against the unfixed CSS:

Error: expect(locator).toBeInViewport() failed
Expected: in viewport
Received: viewport ratio 0

The fix is one line: #grid-table td.cell-invalid { overflow: visible; }.

How I found it: not from a test — from looking at a screenshot of the feature while regenerating the docs. That lesson is now in CLAUDE.md → Test discipline, alongside the toContain-can't-see-absence and untested-WS-surface notes.

Docs (DoD steps 5–7)

• Regenerated every docs/screenshots/*.png and the README demo.gif — they still showed the v1.1.1 banner, and the splash now advertises DO overtime.
• New docs/screenshots/screenshot-grid-validation.png (the feature actually working — the message reads "STARTTIME: minutes must be a multiple of 15"), a matching capture step in scripts/capture-screenshots.mjs, and a README section for it.
• CHANGELOG.md entry under Fixed.

Test plan

• npx tsc --noEmit clean.
• Confirmed the new assertion fails on the unfixed CSS and passes on the fix.
• npx playwright test tests/grid-validation.spec.ts tests/assistant.spec.ts tests/overtime.spec.ts — 35/35.
• Visually verified the rendered screenshot shows the message.