refactor(registration): reduce RegisterForm complexity by extracting hooks and subcomponents

Problem

packages/web-ui-registration/src/RegisterForm.tsx currently starts with:

/* eslint-disable complexity */

This indicates excessive cyclomatic complexity and mixed concerns in a single component.
The file (~316 lines) currently combines:

• Form validation logic
• API error handling logic
• Settings-based conditional logic
• Large, repetitive JSX blocks
• UI rendering and business logic in one place

This makes the component difficult to read, maintain, and safely modify.

Why this matters

• Disabling the complexity linter hides real technical debt
• The registration flow is a critical user path
• Any future change risks regressions due to tightly coupled logic
• Similar forms (e.g. LoginForm) already follow a more modular pattern

Proposed Solution (Refactor Only – No Behavior Changes)

This issue proposes a pure refactor with zero functional or UI changes, focused on reducing complexity and improving maintainability.

Planned improvements:

1. Extract form validation logic into a custom hook

   • Centralize field validation rules
   • Preserve existing react-hook-form behavior

2. Extract registration error handling logic into a dedicated hook

   • Centralize API error mapping
   • Preserve existing error messages and redirects

3. Reduce JSX duplication by extracting reusable form field components

   • Keep accessibility (aria-*) attributes unchanged
   • Preserve current layout and styling

4. Remove eslint-disable complexity from RegisterForm.tsx

Scope & Guarantees

• No breaking changes
• No new dependencies
• No UX or behavior changes
• Same validation rules and error messages
• Same accessibility attributes
• Refactor only (logic is moved, not rewritten)

Expected Outcome

• RegisterForm.tsx complexity reduced and linter rule re-enabled
• Improved separation of concerns (validation, error handling, UI)
• Cleaner and more maintainable registration code
• Easier future enhancements with lower regression risk

Start with packages/web-ui-registration/src/RegisterForm.tsx and compare its structure with the existing LoginForm pattern. Trace the validation, API error handling, settings conditionals, and repeated JSX before separating them while preserving react-hook-form behavior, messages, redirects, accessibility attributes, layout, and styling. Done means the complexity suppression is removed and the linter passes without behavior changes.