docs: expand UI form answers

[splincode]

Summary

• expand all 28 answers in the Формы и ввод данных subsection
• cover labels, placeholders, validation messages and client/server validation boundaries
• clarify validation timing, backend error mapping, form-level errors and inline validation
• expand progressive disclosure, field reduction and multi-step form design
• cover autosave state, confirmed persistence, irreversible actions, confirmation and undo
• compare save button, autosave and optimistic save through risk and control trade-offs

Scope

• continues Расширить полные ответы во всех разделах #49
• UI/UX-only docs change in src/pages/ui-ux/index.md
• completes the Формы и ввод данных subsection
• accessibility and later UI/UX subsections stay for follow-up PRs

Verification

CI should validate:

• npm run format:check
• production build

Part of #49