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>

playwright.config.ts

@@ -0,0 +1,46 @@
+// ABOUTME: Playwright E2E test configuration for browser-based testing.
+// ABOUTME: Configures Chromium-only headless testing with automatic dev server startup.
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  // Test directory for E2E tests
+  testDir: "./e2e",
+
+  // Run tests in parallel
+  fullyParallel: true,
+
+  // Fail the build on CI if you accidentally left test.only in the source code
+  forbidOnly: !!process.env.CI,
+
+  // Retry failed tests on CI only
+  retries: process.env.CI ? 2 : 0,
+
+  // Limit parallel workers on CI to avoid resource issues
+  workers: process.env.CI ? 1 : undefined,
+
+  // Reporter configuration
+  reporter: [["html", { open: "never" }], ["list"]],
+
+  // Shared settings for all projects
+  use: {
+    // Base URL for navigation actions like page.goto('/')
+    baseURL: "http://localhost:3000",
+
+    // Collect trace on first retry for debugging
+    trace: "on-first-retry",
+
+    // Take screenshot on failure
+    screenshot: "only-on-failure",
+  },
+
+  // Configure projects - Chromium only per requirements
+  projects: [{ name: "chromium", use: { ...devices["Desktop Chrome"] } }],
+
+  // Run dev server before starting tests
+  webServer: {
+    command: "pnpm dev",
+    url: "http://localhost:3000",
+    reuseExistingServer: !process.env.CI,
+    timeout: 120 * 1000, // 2 minutes for Next.js to start
+  },
+});