test: add Firefox E2E project to Playwright config for cross-browser coverage

[sw-factory-automations]

Description

E2E tests currently run only on Chromium. CSS rendering differences, keyboard event handling, and focus behavior vary across browsers. Adding a Firefox project to the Playwright config catches browser-specific regressions before users encounter them.

The recent bug history shows 14 E2E-related bugs in the past analysis period. While most were selector fragility (now fixed via testid migration), browser-specific issues would only surface through user reports today.

Acceptance Criteria

• playwright.config.ts adds a firefox project using devices["Desktop Firefox"] for the main (non-visual-regression) test suite
• Firefox project runs the same test suite as the Chromium project (same testIgnore pattern)
• CI workflow updated to run both Chromium and Firefox projects (can run in parallel)
• Any Firefox-specific test failures are fixed (CSS, focus, keyboard differences)
• Visual regression tests remain Chromium-only (baseline screenshots are browser-specific)
• pnpm lint && pnpm typecheck && pnpm test pass
• pnpm test:e2e passes on both Chromium and Firefox

Dependencies

None

Technical Notes

• Add to the projects array in playwright.config.ts: { name: "firefox", testIgnore: /visual-regression/, use: { ...devices["Desktop Firefox"] } }
• Common Firefox differences to watch for: focus-visible behavior, scrollIntoView timing, drag-and-drop event sequence, contenteditable caret positioning
• If specific tests are flaky on Firefox due to known Playwright/Firefox issues, use test.skip with a comment linking to the upstream issue — don't disable the entire Firefox project
• CI may need increased timeout or parallelism adjustment since test count doubles
• Consider running Firefox only on the main branch or PR merge (not on every push) if CI time is a concern

[sw-factory-automations]

Approval Required

This is a HIGH risk change:

• Doubles CI E2E runtime (381 tests × 2 browsers)
• May surface Firefox-specific failures that require code changes to fix
• CI cost and time implications need product owner sign-off

Comment "approved" to release this to the automation queue.