Add Playwright E2E testing infrastructure

- Add playwright-web-flake to flake.nix for NixOS browser support
- Pin @playwright/test@1.56.1 to match nixpkgs version
- Create playwright.config.ts with Chromium-only, auto-start dev server
- Add e2e/smoke.spec.ts with initial smoke tests
- Add .mcp.json for Claude browser control via MCP
- Update .gitignore for playwright artifacts
- Remove E2E test skip from spec.md Known Limitations
- Update specs/testing.md to require three-tier testing approach

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

spec.md

@@ -835,7 +835,7 @@ The following are **out of scope** for MVP:
 | Hormonal birth control | May disrupt natural cycle phases |
 | API versioning | Single version; breaking changes via deprecation |
 | Formal API documentation | Endpoints documented in spec only |
-| E2E tests | Unit + integration tests only (authorized skip) |
+| Multi-user support | Single-user design only |
 
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