feat: add per-gallery thumbnail scale setting (#172) (#251)

Add a thumbnailScale field (xs/sm/md/lg/xl) to gallery layout settings
that adjusts column counts for Grid, Masonry (columns mode), and Mosaic
layouts. Each scale maps to a column offset applied on top of the
layout's base columns, letting photographers control photo density.

- Add thumbnailScale to GalleryLayoutSettings type
- Apply scale offset in Grid, Masonry, and Mosaic layout components
- Add thumbnail scale dropdown to admin theme customizer
- Conditionally show dropdown only for applicable layouts
- Safelist dynamic grid-cols classes in Tailwind config
- Add i18n keys for EN, DE, PT, RU locales

Co-authored-by: Paul Nothaft <[email protected]>

frontend/src/types/theme.types.ts

@@ -47,6 +47,9 @@ export interface GalleryLayoutSettings {
   
   // Mosaic specific
   mosaicPattern?: 'random' | 'structured' | 'alternating';
+
+  // Thumbnail scale (applies to Grid, Masonry columns, Mosaic)
+  thumbnailScale?: 'xs' | 'sm' | 'md' | 'lg' | 'xl';
 }
 
 export interface ThemeConfig {