fix: add focus trap, keyboard handling, and aria-modal to PolicyConflictDialog (Closes #297)

[waterWang]

Summary

Fixes the accessibility issues in PolicyConflictDialog as described in #297.

Changes

Component (components/ui/policy-conflict-dialog.tsx)

• Added aria-modal="true" to the dialog container
• Added ref onto the dialog container for focus trap targeting
• Integrated the existing useFocusTrap hook from lib/hooks/use-focus-trap.ts:
  • Auto-focus on mount: Focus moves to the first focusable element (Cancel button) when the dialog opens
  • Focus trap: Tab/Shift+Tab cannot move focus to elements outside the dialog while it's open
  • Escape key: Pressing Escape calls onCancel and closes the dialog
  • Focus restoration: When the dialog closes, focus is restored to the element that had focus before the dialog opened

Tests

All 10 existing tests in test/policy-conflict-dialog-a11y.test.tsx pass, including the aria-modal="true" assertion.

Acceptance Criteria

• Opening the dialog moves keyboard focus into it
• Tab/Shift+Tab cannot move focus to elements outside the dialog while it's open
• Pressing Escape calls onCancel and closes the dialog
• Closing the dialog (via any button or Escape) restores focus to the triggering element
• aria-modal="true" is present
• All tests pass under npm test

Closes #297