fix: resolve tri-panel interaction overlap and add browser regression checks

[admmpy]

Summary

• fix tri-panel layout hit-testing/stacking boundaries so question tabs and content controls are reachable/clickable
• constrain workspace and editor pane overflow to prevent cross-pane pointer overlap
• add Vitest browser-mode config + Chromium regression tests for tab switching and hint reveal interactions

Validation

• cd frontend && npm run lint
• cd frontend && npm run build
• cd frontend && npm test
• cd frontend && npm run test:browser

Risk

• low-to-moderate CSS layout risk across breakpoints due to new overflow/stacking rules
• mitigated with browser regression checks and manual Playwright verification on real generate/solution/hint flows