Fix page scroll blocked when hovering over terminal demo

[to-be-coder]

Summary

• The wheel event handler on the canvas was unconditionally calling e.preventDefault(), which blocked native page scrolling whenever the mouse was over a gridland canvas (e.g. the terminal demo on the docs site)
• Removed the preventDefault() call and switched to a passive wheel listener, so page scroll works normally while TUI scroll events still dispatch to renderables

Future consideration

If a component needs to own scroll (e.g. a scrollable list or chat window), it should opt in by calling event.preventDefault() on the TUI MouseEvent in its scroll handler. The browser-renderer can then check tuiEvent.defaultPrevented and conditionally block page scroll. This keeps the default behavior permissive and avoids breaking page scroll for components that don't need it.

Test plan

• Open docs site, hover over the terminal demo window, scroll — page should scroll normally
• Verify any TUI components with scroll handlers (e.g. chat) still receive scroll events

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