feat: standardize FieldShell accessibility boundary (AEG-V16-017)

Centralize label, error, help, required, and ARIA relationships across core field wrappers. Preserve WBS evidence and keep the item IN_PROGRESS pending predecessor acceptance evidence.

Evidence: frontend pnpm typecheck; pnpm test (42/42); pnpm build.

frontend/src/shared/ui/components/KsTextField.vue

@@ -1,34 +1,25 @@
 <script setup lang="ts">
-import { computed, useId } from 'vue'
 import { useUiAdapter } from '../adapter/useUiAdapter'
+import FieldShell from './FieldShell.vue'
 
 const props = defineProps<{ modelValue: string; label: string; inputId?: string; disabled?: boolean; required?: boolean; error?: string; help?: string; placeholder?: string }>()
 const emit = defineEmits<{ 'update:modelValue': [value: string]; blur: [event: FocusEvent] }>()
 const adapter = useUiAdapter()
-const generatedId = useId()
-const resolvedId = computed(() => props.inputId ?? `ks-field-${generatedId}`)
 </script>
 
 <template>
-  <div class="ks-field">
-    <label :for="resolvedId">{{ label }} <span v-if="required" aria-hidden="true">*</span></label>
+  <FieldShell :label="label" :input-id="inputId" :required="required" :error="error" :help="help" v-slot="field">
     <component
       :is="adapter.components.TextField"
-      :input-id="resolvedId"
+      :input-id="field.inputId"
       :model-value="modelValue"
       :disabled="disabled"
-      :invalid="Boolean(error)"
+      :invalid="field.invalid"
       :placeholder="placeholder"
-      :aria-describedby="error || help ? `${resolvedId}-message` : undefined"
+      :aria-describedby="field.describedBy"
+      :aria-required="field.required || undefined"
       @update:model-value="emit('update:modelValue', $event)"
       @blur="emit('blur', $event)"
     />
-    <small v-if="error || help" :id="`${resolvedId}-message`" :class="{ 'ks-danger-text': error }">{{ error ?? help }}</small>
-  </div>
+  </FieldShell>
 </template>
-
-<style scoped>
-.ks-field { display: grid; gap: var(--ks-space-1); }
-label { font-weight: 650; }
-small { color: var(--ks-color-neutral-600); }
-</style>