feat: add keyboard shortcuts help overlay

[vku2018]

Summary

• add a shared ShortcutsHelp client component that opens from ? / Shift+/ and ignores editable fields
• render the overlay as a named modal dialog with Escape, close-button, and backdrop dismissal
• mount the overlay once from src/app/layout.tsx so it is available on every route
• document the shortcut in the README

Closes #148.

Validation

• PASS: npm test -- ShortcutsHelp --runInBand (7 tests)
• PASS: npm run lint
• PASS: git diff --cached --check
• BASELINE FAIL: npm run build fails on src/app/quote/Client.tsx:41 with Cannot find name 'assetsDiffer'
• BASELINE FAIL: npm test -- --runInBand fails only in src/app/quote/page.test.tsx with ReferenceError: assetsDiffer is not defined; 22 suites and 111 tests pass