Create global, standard "isMobile" utility

[KevinWu098]

There's roughly 50 instances of "isMobile" across 14 files and they're all of vaguely different implementations. Most refer to screen width, some at 750px and others at 960px. Others refer to the actual device.

In short, create or add to an existing utilities file and standardize the breakpoints.

[ronaldwen07]

I've submitted a PR to fix this issue: ronaldwen07#2

The changes standardize all mobile breakpoint detection to use the useIsMobile() hook with the theme's 'sm' breakpoint (768px), removing the inconsistent 960px MOBILE_BREAKPOINT constant.

Updated 11 files to ensure consistent mobile detection across the application.