refactor: centralize UI config and inject CSS variables dynamically

src/components/Header.vue

@@ -65,7 +65,10 @@ onMounted(() => {
   /* Remove hardcoded colors and use theme variables */
   background: var(--header-bg);
   color: var(--text-color);
-  padding: 1rem;
+  height: var(--header-height);
+  padding: 0 1rem;
+  display: flex;
+  align-items: center;
   /* box-shadow handled by glass-panel class */
   position: sticky;
   top: 0;
@@ -77,7 +80,7 @@ onMounted(() => {
 }
 
 .header-content {
-  max-width: 100%;
+  width: 100%;
   padding: 0;
   margin: 0 auto;
   display: flex;