fix: rename Open to View Details and hide empty Details section (refs #43)

- Rename "Open" button to "View Details" on desktop and mobile document lists
- Add hasDisplayableMetadata helper to check if document has metadata to display
- Conditionally render Details section only when metadata exists
- Prevents showing empty "Details" header for documents without metadata

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/documents/mobile/DocumentsMobileScreen.tsx

@@ -200,7 +200,7 @@ export const DocumentsMobileScreen: React.FC = () => {
                     </button>
                   </div>
                   <div className="flex gap-2 items-center flex-wrap">
-                    <Button onClick={() => navigate(`/garage/documents/${doc.id}`)}>Open</Button>
+                    <Button onClick={() => navigate(`/garage/documents/${doc.id}`)}>View Details</Button>
                     <Button onClick={() => triggerUpload(doc.id)}>Upload</Button>
                     {upload.isPending && currentId === doc.id && (
                       <span className="text-xs text-slate-500">{upload.progress}%</span>