[codex] Fix Firefox message reply overflow

[guaka]

What changed

• Bounds the message thread flex layout so Firefox can shrink the message list instead of pushing the reply composer below the viewport.
• Keeps the reply editor internally scrollable after a capped height while leaving the send controls visible.
• Adds a targeted Firefox Playwright project and regression test for the long-reply overflow case.
• Updates the e2e browser setup so Firefox is available in CI alongside Chromium.

Fixes #2722

Validation

• npm run test:client -- ThreadReply
• bash -n scripts/e2e/test-e2e.sh
• npx eslint modules/messages/client/components/Thread.component.js modules/messages/client/components/ThreadMessages.js modules/messages/client/components/ThreadReply.js playwright.config.js tests/e2e/test.js tests/e2e/messages-layout.spec.js
• TRUSTROOTS_E2E_SKIP_COVERAGE_REPORT=true npm run test:e2e -- --project=messages --project=messages-firefox-layout --grep "reply editor remains usable"

Notes

• The full e2e suite was also attempted. The new messages-firefox-layout test and the existing messages project passed, but the full run ended with unrelated public/auth/member failures and later ENOSPC: no space left on device errors after generating large Playwright artifacts locally.