Introduce a shared accessible Dialog primitive and fix focus restoration across all modals

[1nonlypiece]

📌 Description

The accessibility audit (design/accessibility-audit/findings/05-modals.md,
finding F-05-01, severity Critical, WCAG 2.4.3) documents that modals do not
restore focus to their trigger on close, and that focus/trap behavior is
re-implemented inconsistently per modal because there is no shared Dialog
primitive. Affected surfaces include CommitmentCreatedModal.tsx,
CommitmentDetailsModal.tsx, SettlementModal.tsx, and
CommitmentEarlyExitModal/.

This issue introduces a shared accessible Dialog and migrates modals onto it.

Goal: one tested Dialog primitive that traps focus, closes on Esc,
restores focus to the trigger, and is reused by every modal.

🎯 Requirements and Context

• Build a Dialog that: captures document.activeElement on open, traps focus
  while open, closes on Esc and backdrop click, and calls
  previousActiveElement.focus() on close.
• Apply correct ARIA (role="dialog", aria-modal, labelled title).
• Migrate CommitmentCreatedModal.tsx, CommitmentDetailsModal.tsx,
  src/components/modals/SettlementModal.tsx, and CommitmentEarlyExitModal.
• Satisfy the acceptance criteria in
  design/accessibility-audit/acceptance-criteria.md and
  component-spec-updates.md.

🛠️ Suggested Execution

1. Create a branch

git checkout -b feature/shared-dialog-primitive

2. Implement changes

• Add src/components/ui/Dialog.tsx (+ module CSS).
• Migrate the modals listed above onto it.
• Add src/components/__tests__/Dialog.test.tsx covering focus trap, Esc,
  backdrop, and focus restoration; update affected modal tests.

3. Test and commit

• Run npm test.
• Edge cases: no focusable children, nested modals, Esc vs backdrop close,
  trigger removed before close.

Example commit message

fix: shared accessible Dialog primitive with focus restoration (F-05-01)

✅ Guidelines

• Minimum 95% test coverage on the Dialog and migrated paths.
• Clear documentation of the primitive's a11y contract.
• Timeframe: 96 hours.

🏷️ Labels

type-security · type-refactor · area-frontend · MAYBE REWARDED · GRANTFOX OSS · OFFICIAL CAMPAIGN

💬 Community & Support

• Join the CommitLabs contributor Discord: https://discord.gg/WV7tdYkJk
• Introduce yourself before starting to avoid duplicate work.
• Maintainers triage actively and review fast.