Add token auto-refresh, single-transaction endpoint, shift summary, and CI

- client/api/client.ts: shared refreshPromise prevents concurrent refresh races;
  dispatches auth:logout event when refresh fails
- client/context/AuthContext.tsx: listen for auth:logout to clear user state
- server/routes/transactions.ts: POST / real-time single transaction through
  payment abstraction (201 completed, 202 pending); GET /reports/shift shift
  window totals with averageTransaction, shiftOpen/shiftClose timestamps
- .github/workflows/ci.yml: server typecheck+build, client typecheck+build,
  Docker smoke-test on push/PR to main

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,75 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  server:
+    name: Server — typecheck & build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: server
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+          cache-dependency-path: server/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Generate Prisma client
+        run: npx prisma generate
+
+      - name: Typecheck
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npm run build
+
+  client:
+    name: Client — typecheck & build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: client
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+          cache-dependency-path: client/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Typecheck
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npm run build
+
+  docker:
+    name: Docker build (smoke test)
+    runs-on: ubuntu-latest
+    needs: [server, client]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build image
+        run: |
+          docker build \
+            --build-arg NODE_ENV=production \
+            -t vendor-pos:ci .