No browser-level smoke tests

[EdmondDantes]

Chore

Board, dock and theme interactions are verifiable only by hand — there is no browser automation in the repository, so npm run build gates on unit tests that cover HTML escaping and the API boundary and nothing that renders.

Sketch

• A Playwright setup wherever a browser exists, covering the interactions a unit test cannot see: dragging a card, opening the dock, switching theme, the keyboard paths that dev/DECISIONS.md already treats as a contract.
• Keep it out of the default npm test if the environment has no browser; a separate script is fine.

Acceptance

• A smoke run opens the board against a stub API and asserts the dock, the theme switch and the keyboard navigation still work.