test: add login journey, dashboard row data, and error message tests

[kashish2508]

Adds 3 new Playwright E2E tests covering login-to-dashboard journey, dashboard data integrity, and login error message verification.

Test Coverage Additions

1. Login-to-Dashboard E2E Journey

• Gap identified — Existing login tests verified the redirect URL but never checked that dashboard content actually rendered after arrival.
• User behavior — User fills login form with valid credentials, sees welcome message, gets redirected to dashboard, and sees stat cards with data.

2. Dashboard Activity Table Row Data

• Gap identified — Existing test only asserted that table rows exist (count > 0) without verifying actual row content.
• User behavior — User views the dashboard and sees correct activity data: Alice/Created account, Bob/Placed order, Charlie/Updated profile with dates.

3. Login Error Message Text

• Gap identified — Existing empty-form test checked the .error CSS class but never asserted the specific error message text.
• User behavior — User submits an empty login form and sees the message "Username and password required".

Test files

• tests/login.spec.ts (modified — 1 new test, 1 enhanced test)
• tests/dashboard.spec.ts (modified — 1 new test)
• tests/specs/test-plan.md (updated with 3 new test entries)

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