Implement basics for E2E tests driven by playwright

[zoldar]

Changes

This PR implements basis for running end-to-end dashboard tests using playwright.

• A distinct e2e_test MIX environment is introduced for running the tests
• The end-to-end tests live under ./e2e directory, with playwright and all dependencies installed directly there
• A test specific seeds file is put under ./priv/repo/e2e_seeds.exs - it might be subject to change if we decide to change out approach to setting up system state for tests - it's being discussed internally
• End-to-end tests are run as a part of CI pipeline; screenshots for failed tests uploaded to GH artifacts

The tests can be run locally in two ways:

• mix test.e2e - standard mode outputting test report in the terminal
• mix test.e2e.ui - interactive mode running in a dedicated window, with ability to run tests and review the outputs selectively

When run on CI, the tests are sharded (just two shards for now).

Failed test report for a single shard:

https://github.com/plausible/analytics/actions/runs/21710287043/job/62611828894

Full test report summary available under merge-sharded-e2e-test-report job (after expanding "Merge into list report"):

https://github.com/plausible/analytics/actions/runs/21710287043/job/62612031552

Screenshots from failed tests on CI are available as downloadable artifacts from CI worflow summary page, like this one:

https://github.com/plausible/analytics/actions/runs/21710287043

While we could work towards fancier report output, I'd opt for keeping things simple for now and not adding en explicit report the the PR thread just yet, to reduce noise.

There are only a couple example tests for now. Follow-up work will involve expanding it into a more comprehensive set of test scenarios.