feat: add justified/rows layout mode to masonry gallery (#146)

Add Google Photos-style justified row layout as a mode within masonry:

- Add masonryMode setting: 'columns' (Pinterest) or 'rows' (Google Photos)
- Create justifiedLayoutCalculator utility for row-based layouts
- Extract and store image dimensions on upload for layout calculations
- Include width/height in gallery API response
- Add row height and last row behavior controls to theme customizer
- Support responsive container width detection with ResizeObserver

Photos in rows mode maintain their aspect ratios while filling
horizontal rows at a consistent height. The number of photos per
row is automatically calculated based on target row height and
photo dimensions.

Closes #146

frontend/src/components/admin/ThemeEditorModal.tsx

@@ -22,7 +22,8 @@ const layoutIcons: Record<GalleryLayoutType, React.ReactNode> = {
   carousel: <Play className="w-4 h-4" />,
   timeline: <Clock className="w-4 h-4" />,
   hero: <Image className="w-4 h-4" />,
-  mosaic: <LayoutGrid className="w-4 h-4" />
+  mosaic: <LayoutGrid className="w-4 h-4" />,
+  justified: <Layers className="w-4 h-4" />
 };
 
 export const ThemeEditorModal: React.FC<ThemeEditorModalProps> = ({