Support selected-area capture on mobile touch screens

[sudo-eugene]

Closes #179

What changed

This makes selected-area capture usable on touch screens by letting finger drags draw the selection rectangle, while preserving the existing desktop mouse and Escape-key behavior.

On mobile, the area-picker prompt now stays below the phone safe area and uses an inline Cancel link instead of desktop-only Escape instructions.

Why this shape

The important constraint is that the prompt must not intercept selection gestures. Making the whole prompt tappable would make the new mobile Cancel control easy to wire up, but it would also block drags that start over the prompt text. This keeps the prompt itself pass-through and only lets the inline Cancel link receive taps.

The drag handling uses pointer events instead of adding a separate touch-only path. That keeps mouse, pen, and touch input on the same behavior path and avoids maintaining two versions of the same selection logic.

Test plan

• npx prettier --check src/widget/area-picker.ts e2e/widget.spec.ts e2e/widget.live.spec.ts test/setup.ts
• LIVE_TARGET=local PLAYWRIGHT_BASE_URL=http://localhost:8788 CI=true npx playwright test --project=chromium --shard=1/2
• LIVE_TARGET=local PLAYWRIGHT_BASE_URL=http://localhost:8788 CI=true npx playwright test --project=chromium --shard=2/2
• make check
• make test
• make build-all

[neonwatty]

Reviewed with three independent sub-agents. Fixed the actionable findings by tightening coarse-pointer behavior, using a semantic mobile Cancel button with a minimum touch target, clearing test localStorage per test, and strengthening area-picker cancellation/touch E2E coverage. PR CI is green.

[github-actions]

🎉 This PR is included in version 1.37.3 🎉

The release is available on GitHub release

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