Refactor browser fixes and optimize scroll reset behavior for perform… - #6105

Checklist

• I have read the [Contribution Guidelines](https://github.com/SillyTavern/SillyTavern/blob/release/CONTRIBUTING.md).

Summary

This branch contains the changes from commit 4c4d9258. It includes two targeted performance and correctness fixes:

Changes

public/scripts/browser-fixes.js

• Removed the legacy mobile viewport workaround that repeatedly toggled document.documentElement.style.position.
• This caused unnecessary layout/reflow work during resize events, including Android soft-keyboard transitions.
• Removed the now-unused isMobile import.

public/scripts/keyboard.js

• Added a module-level WeakSet to applyScrollResetBehavior.
• Prevents duplicate focusout listeners from being attached when the function is called repeatedly during DOM/class updates.
• Uses WeakSet so removed containers can still be garbage-collected.

Why

• Reduces keyboard and orientation-change jank.
• Prevents listener accumulation during long sessions.
• Preserves existing scroll-reset behavior.

Testing

• Tested keyboard open/close and device rotation on Android/Chrome.
• Verified scroll-reset containers still reset scrollTop/scrollLeft correctly.
• Confirmed there are no remaining isMobile references in browser-fixes.js.

Notes

The original commit was force-removed from release (origin/release is now at 8172dcd0) and is maintained on perf/mobile-chat-input-optim. No other release changes are included in this PR.