feat(contracts): Multi-signer e-signature system with PDF overlay

- Add multi-signer contract creation with staff/customer selection
- Add signer-specific signing URLs (/sign/s/{token})
- Add sequential and parallel signing modes
- Fix can_sign check in frontend (use top-level response field)
- Fix required field validation to work for all template types
- Fix PDF overlay generation for HTML templates with generated PDFs
- Add signature pre-fill support in template field editor
- Add signature remember/re-apply feature during signing
- Update PDFOverlayService to read from contract.original_pdf_path

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/TESTING.md

@@ -0,0 +1,50 @@
+# Testing Guide
+
+## Overview
+
+This project uses two testing frameworks:
+- **Vitest** for unit and integration tests (src/__tests__)
+- **Playwright** for end-to-end tests (tests/e2e/)
+
+## Running Tests
+
+### Unit/Integration Tests (Vitest)
+
+```bash
+# Run all unit tests
+npm run test
+
+# Run tests in watch mode (auto-rerun on changes)
+npm test
+
+# Run tests with coverage report
+npm run test:coverage
+
+# Run tests with UI (interactive mode)
+npm run test:ui
+```
+
+### End-to-End Tests (Playwright)
+
+```bash
+# Run all E2E tests
+npm run test:e2e
+
+# Run E2E tests with UI
+npm run test:e2e:ui
+
+# Run E2E tests in headed mode (see browser)
+npm run test:e2e:headed
+```
+
+## App Tenant Validation Tests
+
+Location: `src/__tests__/App.tenant.test.tsx`
+
+Tests the critical subdomain-based tenant validation logic. Run with:
+
+```bash
+npm test -- App.tenant
+```
+
+See `src/__tests__/README.md` for detailed testing documentation.