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/KbxSectionHeader.vue

@@ -0,0 +1,82 @@
+<script setup lang="ts">
+/**
+ * KBX Section Header — v60 simplified
+ * Standard header for screen sections
+ */
+withDefaults(
+  defineProps<{
+    title: string
+    count?: number
+    countUnit?: string
+    description?: string
+  }>(),
+  { countUnit: '건' }
+)
+</script>
+
+<template>
+  <header class="kbx-section-header">
+    <div>
+      <h2>
+        {{ title }}
+        <span v-if="count != null"> {{ count.toLocaleString('ko-KR') }}{{ countUnit }}</span>
+      </h2>
+      <p v-if="description">{{ description }}</p>
+    </div>
+    <div class="kbx-section-header__actions">
+      <slot name="actions" />
+    </div>
+  </header>
+</template>
+
+<style scoped>
+.kbx-section-header {
+  min-height: 44px;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+  border-bottom: 1px solid var(--kbx-color-border, #e5e7eb);
+  padding: 0 16px;
+}
+
+.kbx-section-header h2 {
+  margin: 0;
+  font-size: 16px;
+  font-weight: 600;
+  color: var(--kbx-color-text, #000);
+}
+
+.kbx-section-header h2 span {
+  font-size: 13px;
+  color: var(--kbx-color-text-muted, #6b7280);
+  font-weight: 500;
+  margin-left: 6px;
+}
+
+.kbx-section-header p {
+  margin: 4px 0 0;
+  color: var(--kbx-color-text-muted, #6b7280);
+  font-size: 12px;
+}
+
+.kbx-section-header__actions {
+  display: flex;
+  gap: 8px;
+}
+
+@media (prefers-color-scheme: dark) {
+  .kbx-section-header {
+    border-bottom-color: #374151;
+  }
+
+  .kbx-section-header h2 {
+    color: #f9fafb;
+  }
+
+  .kbx-section-header h2 span,
+  .kbx-section-header p {
+    color: #d1d5db;
+  }
+}
+</style>