feat: move unit search input inside the converter card

- Relocate SearchUnits from a standalone position to inside the main converter card
- Add a subtle border-b separator for visual clarity within the card content

components/units/converter/MainConverter.tsx

@@ -118,11 +118,6 @@ export default function MainConverter() {
         onSelectUnit={handleSearchSelect}
       />
 
-      {/* Search */}
-      <div className="flex justify-center">
-        <SearchUnits onSelectUnit={handleSearchSelect} />
-      </div>
-
       {/* Main Converter Card */}
       <Card>
         <CardHeader>
@@ -146,7 +141,12 @@ export default function MainConverter() {
             </div>
           </div>
         </CardHeader>
-        <CardContent className="space-y-4">
+        <CardContent className="space-y-6">
+          {/* Quick Search */}
+          <div className="flex justify-center pb-2 border-b">
+            <SearchUnits onSelectUnit={handleSearchSelect} />
+          </div>
+
           <div className="flex gap-2 items-end">
             <div className="flex-1">
               <label className="text-sm font-medium mb-2 block">Value</label>