ci(e2e): restore green main after report-branding cross-browser regression

Problem

main is persistently red in the required build-e2e job. At current main 0a8c9a49d4215d1756a0a13704d250ef84e612a9, every prerequisite and build step succeeds and the failure is confined to the all-browser E2E test step.

The last green main run was at 9aca3a9068d40fcb17fdb0f10b4c07be0b6d16ec. The first persistent red run followed merge of PR #348 at 3b4802595523474357f3342486489a7c22606071, which added tests/e2e/report-branding.e2e.ts. PR CI runs the full suite only in Chromium and its selected Firefox/WebKit subset does not include report-branding.e2e.ts.

A targeted current-main run of tests/e2e/report-branding.e2e.ts reproduced a Firefox startup/navigation timeout twice before passing on retry, confirming the new test is not deterministic cross-browser. Local WebKit could not be evaluated because this CodexPro host lacks Playwright WebKit system libraries; GitHub Actions remains authoritative for WebKit.

Scope

• Inspect the failing current-main Actions log/artifacts and identify every failing test in the all-browser step before changing code.
• Reproduce the relevant failure or failures on the affected browsers.
• Fix the test or product behavior at the narrowest correct layer; do not mask a real regression with retries or inflated timeouts.
• If #348/report-branding is the cause, make its cross-browser coverage deterministic and add it to the PR Firefox/WebKit subset so the same class of failure is caught before merge.
• If the authoritative log proves a different test is responsible, narrow this issue to that root cause before implementation and keep the report-branding flake as a separate focused follow-up.

Acceptance criteria

• Current-main-equivalent Playwright coverage is green across Chromium, Firefox, and WebKit.
• The previously failing tests pass repeatedly without relying on extra retries or inflated timeouts.
• PR CI exercises the regression path on the browsers that exposed it.
• Required CI is green on the exact PR head.

Inspect the current-main Actions log and artifacts first, then run tests/e2e/report-branding.e2e.ts on the affected browsers to identify every failure. Trace the responsible test or product behavior at the narrowest layer, and review the PR CI browser subset. Done means repeated green Playwright coverage on Chromium, Firefox, and WebKit without extra retries or inflated timeouts.