refactor: remove unused border.scss and update styles

Remove the deprecated border.scss stylesheet and its import from the main SCSS entry file. Replace all legacy border utility class usages with explicit border classes:
- Replace border-ff with border border-white
- Replace border-F1F5F9 with border border-[#f1f5f9]
- Replace border-bottom with border-b border-[#e5e8ee]
- Replace border-top-8 with border-t-8 border-[#f5f5f5]
- Update border-none button styles to add after:!hidden to retain original pseudo-element hiding behavior

src/static/scss/border.scss

@@ -1,31 +0,0 @@
-.border-top {
-  border-top: 1px solid #e5e8ee;
-}
-
-.border-bottom {
-  border-bottom: 1px solid #e5e8ee;
-}
-
-.border-top-8 {
-  border-top: 8px solid #f5f5f5;
-}
-
-.border-ff {
-  border: 1px solid #fff;
-}
-
-.border-F1F5F9 {
-  border: 1px solid #f1f5f9;
-}
-
-.border-bottom-F1F5F9 {
-  border-bottom: 1px solid #f1f5f9;
-}
-
-.border-none {
-  border: none;
-
-  &::after {
-    display: none !important;
-  }
-}

src/static/scss/index.scss

@@ -1,3 +1,2 @@
 @import "./background.scss";
-@import "./border.scss";
 @import "./font-family.scss";