Follow up on #63775: disabling WebKit touch callouts did not fully
prevent iOS Safari from selecting the entire GPUI canvas alongside the
app's text selection.

Remote Web Inspector captured a viewport-sized DOM range containing the
canvas while its computed `-webkit-touch-callout` was `none` but
`-webkit-user-select` was `text`. Set both standard and WebKit-prefixed
`user-select` to `none` directly when GPUI creates the canvas, without
relying on the embedding application's stylesheet. This leaves
touch-event handling and the hidden IME textarea unchanged.

Closes DEL-2016

## Validation

- On an iPhone, injecting ordinary inline `-webkit-user-select: none`
prevented the issue; switching it back to `text` reproduced it. No
`!important` was necessary. This tested the equivalent runtime style,
not a rebuilt app containing this commit.
- `cargo fmt -p gpui_web -- --check` passed.
- `RUSTC_BOOTSTRAP=1 cargo check -p gpui_web --target
wasm32-unknown-unknown` passed. The bootstrap flag is required by the
existing `wasm_thread` dependency.

Release Notes:

- Fixed unintended whole-canvas selection when long-pressing in GPUI web
applications on iOS.