docs: add UI/UX audit tracking checklist

[datlechin]

Summary

• Adds UI_UX_AUDIT.md with 34 items across Critical/High/Medium/Low priorities
• 21/34 already resolved in recent commits (948edbf, 85d66df, 5a09f70, d8b328e, 709105f)
• Remaining 13 items are either larger features (Run Selection, font zoom, toast system) or AppKit limitations

Resolved items

• Onboarding localization, empty result state, pagination spinner
• Import button validation, filter preset dedup, AI retry button
• Execution feedback text, connection test label, settings reset
• License error UI, column header tooltips, copy error button, format menu
• Accent shadow, Pro badge on Sync tab, export format descriptions

Test plan

• Review checklist accuracy against current codebase