perf: modular sass architecture (#2052)

- Modularized the Sass architecture to enhance code maintainability and reduce the output file size
- Replaced deprecated `@import` with `@use` / `@forward`

_sass/components/_buttons.scss

@@ -0,0 +1,51 @@
+@use '../abstracts/variables' as v;
+@use '../abstracts/breakpoints' as bp;
+
+#back-to-top {
+  visibility: hidden;
+  opacity: 0;
+  z-index: 1;
+  cursor: pointer;
+  position: fixed;
+  right: 1rem;
+  bottom: calc(v.$footer-height-large - v.$back2top-size / 2);
+  background: var(--button-bg);
+  color: var(--btn-backtotop-color);
+  padding: 0;
+  width: v.$back2top-size;
+  height: v.$back2top-size;
+  border-radius: 50%;
+  border: 1px solid var(--btn-backtotop-border-color);
+  transition: opacity 0.5s ease-in-out, transform 0.2s ease-out;
+
+  @include bp.lg {
+    right: 5%;
+    bottom: calc(v.$footer-height - v.$back2top-size / 2);
+  }
+
+  @include bp.xxl {
+    right: calc((100vw - v.$sidebar-width - 1140px) / 2 + 3rem);
+  }
+
+  @include bp.xxxl {
+    right: calc(
+      (100vw - v.$sidebar-width-large - v.$main-content-max-width) / 2 + 2rem
+    );
+  }
+
+  &:hover {
+    transform: translate3d(0, -5px, 0);
+    -webkit-transform: translate3d(0, -5px, 0);
+  }
+
+  i {
+    line-height: v.$back2top-size;
+    position: relative;
+    bottom: 2px;
+  }
+
+  &.show {
+    opacity: 1;
+    visibility: visible;
+  }
+}