Switch UI components to new design system tokens

Standardizes color values and styling using the new tokens:
- Replaces hardcoded colors with semantic variables
- Updates background/text/border classes for light/dark mode
- Migrates inputs to UInput/USelect/UTextarea components
- Removes redundant style declarations

app/pages/series/[id].vue

@@ -28,17 +28,19 @@
 
     <div v-else>
       <!-- Page Header -->
-      <PageHeader
-        :title="series.title"
-        :subtitle="series.description"
-        theme="purple"
-        size="large"
-      />
+      <PageHeader :title="series.title" theme="purple" size="large" />
 
       <!-- Series Meta -->
       <section class="py-20 bg-[--ui-bg]">
         <UContainer>
           <div class="max-w-4xl mx-auto">
+            <!-- Series Description -->
+            <div v-if="series.description" class="mb-8">
+              <p class="text-lg text-[--ui-text-muted] leading-relaxed">
+                {{ series.description }}
+              </p>
+            </div>
+
             <div class="flex items-center gap-4 mb-8 flex-wrap">
               <span
                 :class="[