Do not attempt to grab pointer on web if unsupported. (#21534)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/21497

## Solution

Mobile web does not support pointer APIs and panics if they are used.
Check if they exist before using them.

## Testing

Tested on iOS Safari using https://github.com/rectalogic/pointerlock

crates/bevy_winit/Cargo.toml

@@ -78,6 +78,7 @@ bevy_android = { path = "../bevy_android", version = "0.18.0-dev", default-featu
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 wasm-bindgen = { version = "0.2" }
 web-sys = "0.3"
+js-sys = "0.3"
 # TODO: Assuming all wasm builds are for the browser. Require `no_std` support to break assumption.
 bevy_app = { path = "../bevy_app", version = "0.18.0-dev", default-features = false, features = [
   "web",

crates/bevy_winit/src/winit_windows.rs

@@ -397,10 +397,29 @@ fn get_current_videomode(monitor: &MonitorHandle) -> Option<VideoModeHandle> {
         .max_by_key(VideoModeHandle::bit_depth)
 }
 
+#[cfg(target_arch = "wasm32")]
+fn pointer_supported() -> Result<bool, ExternalError> {
+    Ok(js_sys::Reflect::has(
+        web_sys::window()
+            .ok_or(ExternalError::Ignored)?
+            .document()
+            .ok_or(ExternalError::Ignored)?
+            .as_ref(),
+        &"exitPointerLock".into(),
+    )
+    .unwrap_or(false))
+}
+
 pub(crate) fn attempt_grab(
     winit_window: &WinitWindow,
     grab_mode: CursorGrabMode,
 ) -> Result<(), ExternalError> {
+    // Do not attempt to grab on web if unsupported (e.g. mobile)
+    #[cfg(target_arch = "wasm32")]
+    if !pointer_supported()? {
+        return Err(ExternalError::Ignored);
+    }
+
     let grab_result = match grab_mode {
         CursorGrabMode::None => winit_window.set_cursor_grab(WinitCursorGrabMode::None),
         CursorGrabMode::Confined => winit_window