test: add dashboard row data and login error text tests

[kashish2508]

Adds two new Playwright E2E tests that close coverage gaps in the dashboard and login suites.

1. Dashboard activity table row data accuracy (tests/dashboard.spec.ts)

• Gap identified: The existing "Recent Activity table with data" test only asserts that rows exist (not.toHaveCount(0)) but never verifies the actual content of each row.
• User behavior: A user navigates to the dashboard and sees the correct activity entries — Alice/Created account, Bob/Placed order, Charlie/Updated profile — with accurate dates.

2. Login error message text content (tests/login.spec.ts)

• Gap identified: The existing "error when submitting empty form" test checks the .error CSS class but never asserts the actual error message text shown to the user.
• User behavior: A user submits the login form without credentials and sees the specific error message "Username and password required".

Test files

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

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