Add E2E tests to cover all the major flow.

[kasunben]

Description:
Introduce end-to-end (E2E) tests to ensure/validate critical user journeys function as expected. This will increase confidence in releases and reduce regressions.

Acceptance Criteria:

• Set up an E2E testing framework (e.g., Playwright or Cypress).
• Cover major flows such as:
• User registration and login
• Board creation, update, and deletion
• Node and edge management
• File uploads and previews
• Saving and loading boards
• Tests should run locally and in CI.
• Provide clear reporting for test results.

Notes:
E2E tests will complement unit/integration tests and help validate the system from the user’s perspective.