Use SCSS as sole CSS source, remove manual theme.css

- Remove manual themes/default/assets/css/theme.css
- SCSS is compiled by scssphp to css_compiled/theme.css
- All nav-tabs, dropdown-menu, dropdown-item overrides in SCSS only
- Override ALL Bootstrap nav-tabs CSS variables and properties

themes/default/assets/scss/theme.scss

@@ -288,27 +288,49 @@ $header-bg-opacity: 1;
     }
 }
 
-// Remove nav-tabs background so it inherits from parent
+// Navigation tabs — completely override Bootstrap nav-tabs styling
 .nav-tabs {
+    --bs-nav-tabs-border-width: 0 !important;
+    --bs-nav-tabs-border-color: transparent !important;
+    --bs-nav-tabs-border-radius: 0 !important;
+    --bs-nav-tabs-link-hover-border-color: transparent !important;
+    --bs-nav-tabs-link-active-color: var(--nav-font) !important;
+    --bs-nav-tabs-link-active-bg: transparent !important;
+    --bs-nav-tabs-link-active-border-color: transparent !important;
     background-color: transparent !important;
     border: none !important;
+    border-bottom: none !important;
     border-radius: 0 !important;
     gap: 0 !important;
 
     .nav-link {
+        margin-bottom: 0 !important;
         background-color: transparent !important;
         border: none !important;
+        border-top-left-radius: 0 !important;
+        border-top-right-radius: 0 !important;
+        border-bottom-left-radius: 0 !important;
+        border-bottom-right-radius: 0 !important;
         color: var(--nav-font) !important;
 
-        &:hover {
+        &:hover, &:focus {
+            isolation: auto;
             background-color: rgba(255, 255, 255, 0.1) !important;
+            border-color: transparent !important;
         }
 
         &.active {
             background-color: rgba(255, 255, 255, 0.2) !important;
+            border-color: transparent !important;
             border-bottom: 2px solid var(--nav-font) !important;
         }
     }
+
+    .dropdown-menu {
+        margin-top: 0 !important;
+        border-top-left-radius: 0 !important;
+        border-top-right-radius: 0 !important;
+    }
 }
 
 // Bootstrap 5 nested dropdown support (dropdown-submenu) — unlimited depth, CSS hover