refactor(forms): extract reusable form utilities and components

- Add getFirstValidationError utility for nested FieldErrors extraction
- Add mergeWithDefaults utilities (deepMergeWithDefaults, type guards)
- Add useValidationErrorHandler hook for toast + tab navigation
- Add FormSelect component with Controller integration
- Add FormTextarea component with register integration
- Refactor AgentTypeForm to use new utilities
- Remove verbose debug logging (now handled by hook)
- Add comprehensive tests (53 new tests, 100 total)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/lib/forms/index.ts

@@ -0,0 +1,30 @@
+/**
+ * Form Utilities and Hooks
+ *
+ * Centralized exports for form-related utilities.
+ *
+ * @module lib/forms
+ */
+
+// Utils
+export { getFirstValidationError, getAllValidationErrors } from './utils/getFirstValidationError';
+export type { ValidationError } from './utils/getFirstValidationError';
+
+export {
+  safeValue,
+  isNumber,
+  isString,
+  isBoolean,
+  isArray,
+  isObject,
+  deepMergeWithDefaults,
+  createFormInitializer,
+} from './utils/mergeWithDefaults';
+
+// Hooks
+export { useValidationErrorHandler } from './hooks/useValidationErrorHandler';
+export type {
+  TabFieldMapping,
+  UseValidationErrorHandlerOptions,
+  UseValidationErrorHandlerReturn,
+} from './hooks/useValidationErrorHandler';