refactor: replace border-box with box-border and clean up unused scss

Remove the unused box-sizing.scss stylesheet and its import from the main SCSS index file.
Update all Vue component template class references from the custom border-box class to the
native box-border utility class to eliminate redundant custom CSS and align with the project's
utility class conventions.

src/static/scss/box-sizing.scss

@@ -1,3 +0,0 @@
-.border-box {
-  box-sizing: border-box;
-}

src/static/scss/index.scss

@@ -17,7 +17,6 @@
 @import "./scroll.scss";
 @import "./width.scss";
 @import "./white-space.scss";
-@import "./box-sizing.scss";
 @import "./min-height.scss";
 @import "./min-width.scss";
 @import "./gap.scss";