Add Playwright e2e tests for multisite WP environment

[imantsk]

Context A comment on PR #289 suggested adding Playwright tests that run on multisite and creating a test for the specific case discussed in that PR.

What to do

• Add Playwright e2e test workflow with multisite wp environment
• Write a test case that reproduces the behaviour covered by PR #289 (sharing snippets with subsites).

Acceptance criteria

• Playwright tests can be executed against a multisite WordPress install.
• Documentation updated (README or tests/ directory) explaining how to run multisite Playwright tests locally and on CI.

References

• Original comment: https://github.com/codesnippetspro/code-snippets/pull/289#issuecomment-3490282536