feat: establish desktop accessibility baseline

[kev1n77]

Summary

• add reusable modal focus trapping/restoration and WAI-ARIA tab keyboard behavior
• make pane separators keyboard operable with bounded numeric semantics
• dynamically preserve a 320 px inspector budget so the 960x620 minimum window never clips horizontally
• add axe-core DOM checks and WCAG AA theme-token contrast regression tests
• update progress and threat-model evidence

Verification

• npm run desktop:check (43 tests)
• npm run desktop:build
• in-app browser at 960x620 and 1440x900: no page/workspace horizontal overflow
• keyboard verification: tab arrows, separator Home/End, dialog focus loop and focus restoration
• light tertiary contrast: 4.82:1 on surface, 4.56:1 on subtle
• dark tertiary contrast: 4.73:1 on surface, 5.02:1 on subtle
• browser console: no warnings or errors