test: add Playwright E2E coverage

[brentthomas248]

Closes #80

Summary:

• Add Playwright configuration and npm scripts for Chromium E2E coverage.
• Cover onboarding modal progression, sidebar tab persistence, agent selection with wallet reachability, and admin passport controls.
• Add a GitHub Actions E2E workflow and exclude E2E specs from Vitest.
• Add sidebar tab accessibility attributes for stable keyboard/test interaction.

Tests:

• npm run test:e2e -- --project=chromium --workers=1
• npm test

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud