refactor: streamline media converter UI and layout

- consolidated file upload and metadata display in single card
- replaced complex FormatSelector with simple shadcn Select component
- inlined all conversion options without toggle display
- restructured layout to 2-column grid matching pastel app pattern:
  - left column: upload and conversion options
  - right column: conversion results
- removed unused components (FileInfo, FormatSelector, ConversionOptionsPanel)
- cleaned up imports and simplified component hierarchy

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

components/media/ConversionPreview.tsx

@@ -274,7 +274,7 @@ export function ConversionPreview({ job, onDownload, onRetry }: ConversionPrevie
 
           {/* Download button */}
           {job.status === 'completed' && job.result && (
-            <Button onClick={handleDownload} className="w-full gap-2">
+            <Button onClick={handleDownload} className="w-full" variant="default" size="lg">
               <Download className="h-4 w-4" />
               Download{' '}
               {generateOutputFilename(job.inputFile.name, job.outputFormat.extension)}