fix: resolve code quality issues and add missing i18n keys (#162, #163)

Add missing i18n translations for hero image focal point picker in both
EN and DE locales. Fix lint errors across touched files: remove unused
imports/variables, replace raw buttons with shared Button component,
eliminate inline styles, extract duplicated backend validation, and
remove dead heroImagePosition type.

frontend/src/types/theme.types.ts

@@ -43,7 +43,6 @@ export interface GalleryLayoutSettings {
   
   // Hero specific
   heroImageId?: number;
-  heroImagePosition?: 'top' | 'center' | 'bottom';
   heroOverlayOpacity?: number;
   
   // Mosaic specific