chore(frontend): upgrade Bootswatch and Bootstrap SCSS to v5.3.3

- Upgraded Bootswatch SCSS source files across all 25 themes (dev/scss/bootswatch/) to v5.3.3
- Upgraded Bootstrap SCSS source files (dev/scss/bootstrap/) to v5.3.3 to match static/js/bootstrap.bundle.min.js
- Updated dev/scss/bootstrap-overrides.scss to import bootstrap/variables-dark
- Verified error-free LibSass compilation for all 26 themes (25 Bootswatch + bootstrap-orj)
- Recompiled static/css/wvc-main.min.css

dev/scss/bootstrap/_grid.scss

@@ -2,6 +2,12 @@
 //
 // Rows contain your columns.
 
+:root {
+  @each $name, $value in $grid-breakpoints {
+    --#{$prefix}breakpoint-#{$name}: #{$value};
+  }
+}
+
 @if $enable-grid-classes {
   .row {
     @include make-row();