Enhance EffectiveGridBuilder to support background slots and improve scheduling logic

Updated the EffectiveGridBuilder to return an EffectiveGrid containing both scheduled and background slots, allowing for better management of overlapping content. Refactored the ResolveDay method to handle background slots separately, ensuring they fill gaps in the schedule appropriately. Adjusted related components, including GridScheduleGenerator and SchedulePlanner, to accommodate the new structure and improve overall scheduling accuracy. Enhanced documentation and comments for clarity.

frontend/src/shared/lib/locales/en.ts

@@ -644,7 +644,7 @@ export const en = {
         CandidatesFiltered: 'The age cap ruled out every candidate',
         BreakLimitExceeded: 'Breaks in an hour exceed the cap',
         GenreShareExceeded: 'Genre share above the norm',
-        FallbackShareExceeded: 'Background share above the norm',
+        FallbackShareExceeded: 'Emergency filler share above the norm',
       },
 
       title: 'Channels',

frontend/src/shared/lib/locales/ru.ts

@@ -644,7 +644,7 @@ export const ru = {
         CandidatesFiltered: 'Возрастной потолок отсёк всех',
         BreakLimitExceeded: 'Врезок в часе больше потолка',
         GenreShareExceeded: 'Доля жанра выше нормы',
-        FallbackShareExceeded: 'Фона в эфире больше нормы',
+        FallbackShareExceeded: 'Аварийного запаса в эфире больше нормы',
       },
 
       title: 'Каналы',