feat: add accessible compare markets modal

[pq198363-ops]

Closes #368

Summary

• Add CompareMarketsModal with an accessible side-by-side comparison of two prediction markets
• Wire a Compare action into the event detail header using the current market and a related comparison market
• Document the visible behavior and accessibility notes
• Fix the existing DialogContentWithFocusReturn useRef reference so the shared dialog module type-checks/lints when imported

Validation

• PASS: node node_modules/jest/bin/jest.js app/components/__tests__/CompareMarketsModal.test.tsx --runInBand
• PASS: node node_modules/eslint/bin/eslint.js app/components/CompareMarketsModal.tsx app/components/__tests__/CompareMarketsModal.test.tsx "app/(dashboard)/events/event-page/EventDetailsClient.tsx" components/ui/dialog.tsx
• PASS: git diff --check
• NOTE: node node_modules/typescript/bin/tsc --noEmit --pretty false still fails on existing unrelated repo errors such as missing ErrorBoundary, shareProfile, wallet modal symbols, badge variant typing, and existing PredictionCard identifiers.

Notes

• The modal uses the shared Radix dialog wrapper for focus trapping and Escape handling.
• The comparison layout stacks on small screens and switches to two columns on wider screens.

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