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>

specs/testing.md

@@ -6,7 +6,7 @@ When I make changes to the codebase, I want automated tests to catch regressions
 
 ## Testing Strategy
 
-PhaseFlow uses **unit and integration tests** with Vitest. End-to-end tests are not required for MVP (authorized skip).
+PhaseFlow uses a **three-tier testing approach**: unit tests, integration tests, and end-to-end tests.
 
 ### Test Types
 
@@ -14,6 +14,7 @@ PhaseFlow uses **unit and integration tests** with Vitest. End-to-end tests are
 |------|-------|-------|----------|
 | Unit | Pure functions, utilities | Vitest | Colocated `*.test.ts` |
 | Integration | API routes, PocketBase interactions | Vitest + supertest | Colocated `*.test.ts` |
+| E2E | Full user flows, browser interactions | Playwright | `e2e/*.spec.ts` |
 
 ## Framework
 
@@ -148,9 +149,96 @@ describe('GET /api/today', () => {
 });
 ```
 
+## End-to-End Tests
+
+Test complete user flows through the browser using Playwright.
+
+### Framework
+
+**Playwright** - Cross-browser E2E testing with auto-waiting and tracing.
+
+**Configuration (`playwright.config.ts`):**
+```typescript
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./e2e",
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  reporter: [["html", { open: "never" }], ["list"]],
+  use: {
+    baseURL: "http://localhost:3000",
+    trace: "on-first-retry",
+    screenshot: "only-on-failure",
+  },
+  projects: [
+    { name: "chromium", use: { ...devices["Desktop Chrome"] } },
+  ],
+  webServer: {
+    command: "pnpm dev",
+    url: "http://localhost:3000",
+    reuseExistingServer: !process.env.CI,
+  },
+});
+```
+
+### Priority Targets
+
+| Flow | Tests |
+|------|-------|
+| Authentication | Login page loads, login redirects, logout works |
+| Dashboard | Shows decision, displays cycle info, override toggles work |
+| Settings | Garmin token paste, preferences save |
+| Calendar | ICS feed accessible, calendar view renders |
+| Period logging | "Period Started" updates cycle |
+
+### Example Test
+
+```typescript
+// e2e/dashboard.spec.ts
+import { expect, test } from "@playwright/test";
+
+test.describe("dashboard", () => {
+  test("shows training decision for authenticated user", async ({ page }) => {
+    // Login first (or use auth state)
+    await page.goto("/");
+
+    // Verify decision card is visible
+    await expect(page.getByTestId("decision-card")).toBeVisible();
+
+    // Verify cycle day is displayed
+    await expect(page.getByText(/Day \d+ of \d+/)).toBeVisible();
+  });
+
+  test("override toggles update decision", async ({ page }) => {
+    await page.goto("/");
+
+    // Enable flare mode
+    await page.getByLabel("Flare Mode").click();
+
+    // Decision should change to gentle
+    await expect(page.getByText(/GENTLE/)).toBeVisible();
+  });
+});
+```
+
+### NixOS Setup
+
+E2E tests require browser binaries. On NixOS, use `playwright-web-flake`:
+
+```nix
+# In flake.nix inputs
+inputs.playwright-web-flake.url = "github:pietdevries94/playwright-web-flake/1.56.1";
+
+# In devShell
+PLAYWRIGHT_BROWSERS_PATH = "${playwright-driver.browsers}";
+PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD = "1";
+```
+
 ## File Naming
 
-Tests colocated with source files:
+Tests colocated with source files (unit/integration) or in `e2e/` directory (E2E):
 
 ```
 src/
@@ -164,22 +252,33 @@ src/
       today/
         route.ts
         route.test.ts
+e2e/
+  smoke.spec.ts
+  dashboard.spec.ts
+  auth.spec.ts
+  settings.spec.ts
 ```
 
 ## Running Tests
 
 ```bash
-# Run all tests
-npm test
-
-# Run with coverage
-npm run test:coverage
+# Run unit/integration tests
+pnpm test:run
 
 # Run in watch mode
-npm run test:watch
+pnpm test
 
-# Run specific file
-npm test -- src/lib/cycle.test.ts
+# Run E2E tests (headless)
+pnpm test:e2e
+
+# Run E2E tests (visible browser)
+pnpm test:e2e:headed
+
+# Run E2E tests with UI mode
+pnpm test:e2e:ui
+
+# Run all tests (unit + E2E)
+pnpm test:run && pnpm test:e2e
 ```
 
 ## Coverage Expectations
@@ -190,15 +289,20 @@ No strict coverage thresholds for MVP, but aim for:
 
 ## Success Criteria
 
-1. All tests pass in CI before merge
-2. Core decision engine logic has comprehensive tests
+1. All tests (unit, integration, E2E) pass in CI before merge
+2. Core decision engine logic has comprehensive unit tests
 3. Phase scaling tested for multiple cycle lengths
 4. API auth tested for protected routes
+5. Critical user flows covered by E2E tests
 
 ## Acceptance Tests
 
-- [ ] `npm test` runs without errors
+- [ ] `pnpm test:run` runs without errors
+- [ ] `pnpm test:e2e` runs without errors
 - [ ] Unit tests cover decision engine logic
 - [ ] Unit tests cover cycle phase calculations
 - [ ] Integration tests verify API authentication
+- [ ] E2E tests verify login flow
+- [ ] E2E tests verify dashboard displays correctly
+- [ ] E2E tests verify period logging works
 - [ ] Tests run in CI pipeline