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>

INSTRUCTIONS.md

@@ -91,6 +91,7 @@ All endpoints live under `/api/v1`.
 | POST   | /transactions/:id/refund          | manager+      | Refund a completed transaction     |
 | GET    | /transactions/:id/receipt         | Bearer        | Structured receipt payload         |
 | GET    | /transactions/reports/summary     | manager+      | Revenue/tax/top-product summary    |
+| GET    | /transactions/reports/shift       | manager+      | Shift window totals + avg tx value |
 
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