feat: a built-in vector icon set #89

What

Icons with no font or image dependency, drawn from the path primitive so they scale cleanly.

Acceptance

• A built-in icon set as vector paths: play, pause, step, stop, close, chevrons, plus, minus, search, settings, help, pin, the dock drop targets
• Icons are drawn at any size and colour, and inline with text in labels and buttons
• Apps register their own icons as path data in the same format