Add opt-in visual regression checks

[BAKUGOS1]

Goal

Add a basic visual regression workflow that compares screenshots from repeat runs and flags meaningful UI changes.

Scope ideas

• Store baselines outside generated artifacts, or document where users should keep them
• Compare current screenshots against a baseline with a simple threshold
• Report visual differences in Markdown/Excel output
• Keep it opt-in so smoke tests stay fast

Done criteria

• Add a focused implementation or documented spike
• Include a small fixture/test path
• Run npm run quality:gate
• Do not commit generated browser artifacts