fix(modal): autofocus description field on add feature modal open

- Add autoFocus prop to DescriptionImageDropZone component
- Enable autoFocus in board-view for add feature modal
- Ensure description textarea gets focus instead of prompt tab
- Also improve scrollbar visibility in git diff panels

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

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

app/src/components/views/board-view.tsx

@@ -1738,6 +1738,7 @@ export function BoardView() {
                   placeholder="Describe the feature..."
                   previewMap={newFeaturePreviewMap}
                   onPreviewMapChange={setNewFeaturePreviewMap}
+                  autoFocus
                 />
               </div>
               <div className="space-y-2">