test(e2e): stabilize Playwright suite and close plan verification

Make auth/tasks/shifts end-to-end tests deterministic with robust role-aware
fallbacks, single-worker execution, and non-brittle selectors aligned to the
current UI contracts.

Mark verified plan/evidence checklists complete after re-validating backend,
frontend, E2E, security isolation, and infrastructure commands.

frontend/playwright.config.ts

@@ -2,15 +2,17 @@ import { defineConfig, devices } from '@playwright/test';
 
 export default defineConfig({
   testDir: './e2e',
-  fullyParallel: true,
+  fullyParallel: false,
   forbidOnly: !!process.env.CI,
   retries: process.env.CI ? 2 : 0,
-  workers: process.env.CI ? 1 : undefined,
+  workers: 1,
   reporter: 'html',
+  timeout: 60000,
   use: {
     baseURL: 'http://localhost:3000',
     screenshot: 'only-on-failure',
     trace: 'on-first-retry',
+    navigationTimeout: 30000,
   },
 
   projects: [