feat: refactor monolithic App.vue into modular Vue component architecture

- Extract 711-line App.vue into 15+ focused files across 5 directories
- Add data layer (icons, metrics, policies, auditTrail, requests)
- Add composables (useNavigation, useRequests, useChat, useToast)
- Add layout components (SidebarRail, TopBar, FilterBar)
- Add shared components (PanelHead, InfoRow, ToastNotification)
- Add business component (RequestTable) and 5 view components
- Extract global CSS to assets/styles/global.css
- Add start.sh with WSL/Windows cross-platform support
- Add .gitignore for node_modules, dist, and IDE dirs

src/composables/useNavigation.js

@@ -0,0 +1,24 @@
+import { computed, ref } from 'vue'
+import { icons } from '../data/icons.js'
+
+export const navItems = [
+  { id: 'overview', label: '运营总览', icon: icons.dashboard, title: '企业报销智能运营台', desc: '面向财务共享中心的审批、风控、SLA 与智能体协同工作台。' },
+  { id: 'chat', label: '合规对话', icon: icons.message, title: 'AI 合规对话', desc: '上传单据、追问制度依据，并生成可留痕的审核建议。' },
+  { id: 'requests', label: '报销队列', icon: icons.list, title: '报销申请队列', desc: '按风险、补件状态和 AI 建议处理待审单据。' },
+  { id: 'policies', label: '政策规则', icon: icons.file, title: '政策规则中心', desc: '维护差旅、招待、采购和发票校验规则。' },
+  { id: 'audit', label: '审计追踪', icon: icons.audit, title: '审计追踪', desc: '查看关键审批动作、AI 建议和制度命中记录。' }
+]
+
+export function useNavigation() {
+  const activeView = ref('overview')
+
+  const currentView = computed(
+    () => navItems.find((item) => item.id === activeView.value) ?? navItems[0]
+  )
+
+  function setView(view) {
+    activeView.value = view
+  }
+
+  return { activeView, currentView, setView, navItems }
+}