On iOS, WebKit presents a native touch callout when a user long-presses
a canvas. For GPUI Web this selected the full application canvas while
GPUI independently selected the intended word, producing two pairs of
selection handles, a native edit menu, and haptic feedback.

Set `-webkit-touch-callout: none` on the canvas created by `gpui_web`.
GPUI already owns touch gesture recognition on this surface, so WebKit
should not provide a competing canvas interaction.

Tested with:

- `cargo fmt --check --package gpui_web`
- `RUSTC_BOOTSTRAP=1 cargo check -p gpui_web --target
wasm32-unknown-unknown`
- A Delta Web WASM build using the local Zed checkout
- Manual verification on an iPhone that long-press word selection no
longer selects the full canvas

Release Notes:

- N/A