refactor(form-schema): extract schema types and schema-driven behaviors to shared package

Moves formBuilder types, formValidation, useConditionalLogic, useFormSteps,
and formatFieldValue from apps/portal/src to packages/form-schema/src.
Adds @form-schema path alias to both apps/portal and apps/app.
Vue field components remain per-app to allow independent visual evolution.
Behavior-neutral: all 35 Vitest tests green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/portal/tests/unit/formatFieldValue.spec.ts

@@ -1,12 +1,12 @@
 import { describe, expect, it } from 'vitest'
-import { formatFieldValue } from '@/composables/formatFieldValue'
-import { FormFieldType } from '@/types/formBuilder'
+import { formatFieldValue } from '@form-schema/composables/formatFieldValue'
+import { FormFieldType } from '@form-schema/types/formBuilder'
 import type {
   AvailableTag,
   PublicFormField,
   PublicFormSectionOption,
   PublicFormTimeSlot,
-} from '@/types/formBuilder'
+} from '@form-schema/types/formBuilder'
 
 function field(partial: Partial<PublicFormField> = {}): PublicFormField {
   return {