feat: add real-time form validation system

[playmaker410]

Summary

• add Zod-based field and whole-form validation with normalized errors
• add a reusable 300 ms debounced validation hook with touched, dirty, pending, and stale-request handling
• add accessible field, validation-message, valid-state, and form-summary components
• separate polite field announcements from assertive submit summaries and clean up live-region timers
• configure Storybook for Next.js 16 with typed stories for pristine, pending, valid, invalid, form-invalid, async stale-result, and announcement states

The implementation is additive and does not alter the existing domain forms or their runtime error handling.

Verification

• npm test — 679 tests passed across 87 files
• targeted ESLint on all changed source, test, and Storybook files
• npx tsc --noEmit --pretty false
• npm run build
• npm run build-storybook
• git diff --check

Dependency audit note

npm audit --omit=dev continues to report existing advisories in the repository's pinned Next.js, Stellar SDK/Axios, Nano ID, PostCSS, and Sharp dependency chains. Resolving those requires unrelated or breaking upgrades and is intentionally outside this focused change.

Closes #174