Add passkey authentication E2E tests

Uses Playwright's virtual WebAuthn authenticator (CDP) to test:
- Register with passkey + sign in (full round-trip)
- Login fails with no credential (friendly error message)
- Duplicate email rejection
- Duplicate username rejection

Would have caught the credential encoding bug.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

playwright.config.ts

@@ -31,6 +31,14 @@ export default defineConfig({
         baseURL: "http://localhost:3001",
       },
     },
+    {
+      name: "auth",
+      testMatch: "auth.test.ts",
+      use: {
+        ...devices["Desktop Chrome"],
+        baseURL: "http://localhost:3000",
+      },
+    },
     {
       name: "integration",
       testMatch: "integration.test.ts",