Fix touch gesture conflicts in WebGL example apps on mobile

[mel-anthropic]

On touch devices, drag/pan gestures inside the WebGL-based example apps (Maps, ThreeJS, Wiki Explorer) were propagating to the parent scroll view, causing the page to scroll while trying to interact with the 3D content.

Root cause: The rendering libraries these apps use (CesiumJS, Three.js OrbitControls, force-graph) handle drag interaction via pointer events. On touch devices, the browser doesn't know that these pointer event handlers are "consuming" the touch — it still interprets the gesture as a scroll and forwards it to the parent scroll view. The browser only backs off when it sees explicit touch event handlers that signal "I'm handling this touch, don't scroll."

Fix: Add touchstart/touchmove event handlers on the interactive canvas elements that call preventDefault() to signal to the browser that the touch is being handled by the app. Shadertoy already had correct touch handlers, so only a CSS hint was added there.

[pkg-pr-new]

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commit: 37a1c3e