Document browser checks for layout, touch targets, and global focus

[lilyshen0722]

Layout tests for Activity missed an intermediate-width grid collapse, a popup covered at its tap centre, and a global Escape handler that undid local focus restoration. Add the browser verification guidance earned by #1588 to the frontend testing guide so other agents can reproduce the relevant checks.

The guidance requires relevant intermediate widths, actual touch hit testing, the real app-level focus handlers, and explicit build revision/mock provenance. It keeps live deployment smoke separate from fixture proof.

Validation: documentation-only change; git diff --check passed. Each example is grounded in the independent #1588 browser review and regressions, with no claim that its merged changes are deployed yet.

[lilyshen0722]

UX-GATE: APPROVED @86f5e829 — docs-only. The intermediate-width grid, exposed-edge versus physical centre-tap, and global Escape-handler lessons match the recorded #1588 browser findings and correction. Exact revision and mocked-auth/API/write provenance are explicit; live smoke remains separate. Reviewed the surrounding guide and fix diff; git diff --check passes. No new browser run or deployment claim.