feat: move units search and category select to a row below description

- Relocate SearchUnits and Category Select into a dedicated row at the top of MainConverter
- Remove them from the converter card header for a cleaner layout
- Update vertical spacing for better visual hierarchy

components/units/converter/MainConverter.tsx

@@ -111,24 +111,20 @@ export default function MainConverter() {
   }, [selectedUnit]);
 
   return (
-    <div className="w-full space-y-6">
+    <div className="w-full space-y-8">
       {/* Command Palette */}
       <CommandPalette
         onSelectMeasure={setSelectedMeasure}
         onSelectUnit={handleSearchSelect}
       />
 
-      {/* Main Converter Card */}
-      <Card>
-        <CardHeader className="pb-4 border-b">
-          <div className="flex flex-col md:flex-row md:items-center gap-4 justify-between">
-            <CardTitle className="shrink-0">Convert {formatMeasureName(selectedMeasure)}</CardTitle>
-            
-            <div className="flex-1 max-w-md">
+      {/* Quick Access Row */}
+      <div className="flex flex-col md:flex-row md:items-center gap-4 justify-between bg-card p-4 rounded-lg border">
+        <div className="flex-1">
           <SearchUnits onSelectUnit={handleSearchSelect} />
         </div>
 
-            <div className="w-full md:w-48 shrink-0">
+        <div className="w-full md:w-64 shrink-0">
           <Select
             value={selectedMeasure}
             onChange={(e) => setSelectedMeasure(e.target.value as Measure)}
@@ -145,8 +141,13 @@ export default function MainConverter() {
           </Select>
         </div>
       </div>
+
+      {/* Main Converter Card */}
+      <Card>
+        <CardHeader>
+          <CardTitle>Convert {formatMeasureName(selectedMeasure)}</CardTitle>
         </CardHeader>
-        <CardContent className="space-y-6 pt-6">
+        <CardContent className="space-y-6 pt-0">
           <div className="flex gap-2 items-end">
             <div className="flex-1">
               <label className="text-sm font-medium mb-2 block">Value</label>