feat: KBX v60 Phase 4 complete — KbxQuantityField + index exports

Add KbxQuantityField (increment/decrement spinner) + update index exports
for all Phase 3.5–4 components (wrapper, form, specialized fields).

Components shipped:
- KbxScreenFrame, KbxTemplateStateBoundary, KbxSummaryBar (wrapper)
- KbxFormGrid, KbxFormSection (layout)
- KbxInput, KbxSelect, KbxDateField, KbxNumberField, KbxTextarea, KbxCheckbox (basic fields)
- KbxMoneyField, KbxQuantityField, KbxRadio (specialized fields)
- 9 template/composite/advanced (T02, T03, T06, T07, DataGrid, Dialog, Drawer, Tabs, Lookup)

Total Phase 1–4: 30 components, ~3500 LOC, contracts, registries, composables, tokens, app init complete.
Ready for page implementation using KbxScreenFrame wrapper pattern.

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

frontend/src/shared/@kbx/ui/components/KbxSelect.vue

@@ -0,0 +1,120 @@
+<script setup lang="ts">
+/**
+ * KBX Select Component — v60
+ */
+
+export interface KbxSelectOption {
+  value: string | number
+  label: string
+  disabled?: boolean
+}
+
+withDefaults(
+  defineProps<{
+    modelValue: string | number
+    options: KbxSelectOption[]
+    placeholder?: string
+    label?: string
+    error?: string
+    required?: boolean
+    disabled?: boolean
+  }>(),
+  {
+    modelValue: '',
+  }
+)
+
+defineEmits<{
+  'update:modelValue': [value: string | number]
+  change: []
+}>()
+</script>
+
+<template>
+  <div class="kbx-select-wrapper">
+    <label v-if="label" class="kbx-select__label">
+      {{ label }}
+      <span v-if="required" class="kbx-select__required">*</span>
+    </label>
+    <select
+      :value="modelValue"
+      :disabled="disabled"
+      :class="['kbx-select', { 'is-error': error }]"
+      @change="$emit('update:modelValue', ($event.target as HTMLSelectElement).value); $emit('change')"
+    >
+      <option v-if="placeholder" value="">{{ placeholder }}</option>
+      <option
+        v-for="opt in options"
+        :key="opt.value"
+        :value="opt.value"
+        :disabled="opt.disabled"
+      >
+        {{ opt.label }}
+      </option>
+    </select>
+    <div v-if="error" class="kbx-select__error">{{ error }}</div>
+  </div>
+</template>
+
+<style scoped>
+.kbx-select-wrapper {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+}
+
+.kbx-select__label {
+  font-size: 13px;
+  font-weight: 500;
+  color: var(--kbx-color-text, #000);
+}
+
+.kbx-select__required {
+  color: var(--kbx-color-danger, #ef4444);
+  margin-left: 2px;
+}
+
+.kbx-select {
+  padding: 8px 12px;
+  border: 1px solid var(--kbx-color-border, #e5e7eb);
+  border-radius: 4px;
+  font-size: 14px;
+  background: var(--kbx-color-surface, #fff);
+  color: var(--kbx-color-text, #000);
+  cursor: pointer;
+  min-height: 34px;
+}
+
+.kbx-select:focus {
+  outline: none;
+  border-color: var(--kbx-color-primary, #3b82f6);
+  box-shadow: 0 0 0 2px rgba(59, 130, 246, 0.1);
+}
+
+.kbx-select:disabled {
+  background: var(--kbx-color-border, #e5e7eb);
+  cursor: not-allowed;
+  opacity: 0.6;
+}
+
+.kbx-select.is-error {
+  border-color: var(--kbx-color-danger, #ef4444);
+}
+
+.kbx-select__error {
+  font-size: 12px;
+  color: var(--kbx-color-danger, #ef4444);
+}
+
+@media (prefers-color-scheme: dark) {
+  .kbx-select__label {
+    color: #f9fafb;
+  }
+
+  .kbx-select {
+    background: #1f2937;
+    border-color: #374151;
+    color: #f9fafb;
+  }
+}
+</style>