feat: Approval Queue page (T03 transaction template) — third KBX v60 page

Implement ApprovalQueue.vue using KBX Foundation v60 transaction pattern:
- T03 Transaction template for maker-checker approval workflow
- KbxScreenFrame wrapper with breadcrumb/title
- Summary stats (Pending, Approved, Rejected counts)
- Status and action type filters
- Header with model name, action type, status badge
- Request details grid (Request ID, Requester, Requested At, Status)
- Validation metrics display (PBO, DSR, OOS, Target Phase)
- Review & approval section with textarea for comments
- Approve/Reject buttons with submit state
- Review history display (reviewer, date, decision, comment)
- Side panel with request list (fixed position on desktop, stacked on mobile)
- Dark mode and responsive layout

New files:
- features/approval/pages/ApprovalQueue.vue (T03 transaction page)
- features/approval/composables/useApprovalRequests.ts (approval data)
- features/approval/types/index.ts (type definitions)
- features/approval/registry.ts (screen definition)

Demo data: 3 approval requests (pending, approved, rejected) with full workflow.
Mock approval/rejection methods with comment capture.
Ready for API integration and real backend workflow.

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

frontend/src/features/approval/registry.ts

@@ -0,0 +1,48 @@
+/**
+ * Approval Feature Screen Registry
+ */
+
+import type { ScreenDefinition } from '@kbx/contracts'
+
+export const approvalQueueScreen: ScreenDefinition = {
+  screenId: 'governance.approval.queue',
+  title: 'Approval Queue',
+  module: 'ERP',
+  path: '/governance/approvals',
+  component: () => import('./pages/ApprovalQueue.vue'),
+  permissions: ['approval.review'],
+  template: 'T03',
+
+  help: {
+    title: 'Approval Workflow',
+    sections: [
+      {
+        title: 'What is Maker-Checker?',
+        content:
+          'Maker-Checker enforces that critical model decisions require two parties: the requester and an independent reviewer.',
+      },
+      {
+        title: 'How to Approve',
+        content: 'Select a pending request, review the metrics and comments, then approve or reject with your decision.',
+      },
+      {
+        title: 'Decision Criteria',
+        content: 'Activation requires: PBO ≤ 20%, DSR ≥ 95%, OOS ≤ 2.5%, plus 252+ trading-day shadow run.',
+      },
+    ],
+    relatedScreens: ['model-ops.models.list', 'model-ops.shadow-run.queue'],
+  },
+
+  grid: {
+    columnDefs: [
+      { field: 'requestId', headerName: 'Request ID', width: 120 },
+      { field: 'modelName', headerName: 'Model', width: 150 },
+      { field: 'action', headerName: 'Action', width: 100 },
+      { field: 'status', headerName: 'Status', width: 100 },
+      { field: 'requesterName', headerName: 'Requester', width: 120 },
+      { field: 'requestedAt', headerName: 'Date', width: 150 },
+    ],
+  },
+}
+
+export default [approvalQueueScreen]