feat: add quilted layout, fix mosaic, and backfill photo dimensions (#146)

- Add migration to backfill width/height for existing photos without dimensions
- Replace justified masonry mode with quilted layout (mixed sizes based on aspect ratio)
- Rewrite mosaic layout to use proper CSS Grid with span rules
- Fix theme not being applied after gallery login
- Improve columns mode distribution using shortest-column algorithm
- Apply gallery theme regardless of authentication status

frontend/src/types/theme.types.ts

@@ -15,7 +15,7 @@ export interface GalleryLayoutSettings {
   };
   
   // Masonry specific
-  masonryMode?: 'columns' | 'rows' | 'flickr' | 'justified'; // columns = Pinterest-style, rows = custom rows, flickr = Flickr justified-layout, justified = react-photo-album (Google Photos style)
+  masonryMode?: 'columns' | 'rows' | 'flickr' | 'quilted'; // columns = Pinterest-style, rows = justified rows, flickr = Flickr justified-layout, quilted = mixed sizes based on aspect ratio
   masonryGutter?: number;
   masonryRowHeight?: number; // Target row height for rows mode (150-400)
   masonryLastRowBehavior?: 'justify' | 'left' | 'center'; // How to align incomplete last row