## Summary
Fixed the slash menu not appearing when typing `/` on mobile web
browsers.
## Problem
Mobile browsers don't reliably fire keyboard events (`keyDown`) when
using virtual keyboards. This caused the slash menu trigger to fail on
mobile devices.
## Solution
- Changed from handling `keyDown` events to `beforeInput` events
- `InputEvent` is fired consistently across all platforms (mobile and
desktop)
- Added proper handling for IME composition to avoid duplicate triggers
- Uses `waitForUpdate()` to ensure the input is processed before
checking for the trigger
## Test plan
- [x] Tested on mobile Safari (iOS)
- [x] Tested on mobile Chrome (Android)
- [x] Verified desktop browsers still work correctly
- [x] Verified IME input (e.g., Chinese/Japanese) doesn't trigger false
positives
Fixes#12910
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Improved slash menu input handling for better reliability and enhanced
IME (input method editor) composition support.
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