feat: refactor theme handling and update TODO.md

- Replaced references from theme-config to ui-config for better organization and clarity in theme management.
- Updated Solarized icon in ui-config to a pill emoji for improved visual representation.
- Marked the Solarized icon correction task as complete in TODO.md.
- Deleted the now redundant theme-config file to streamline the codebase.

src/actions/preferences.ts

@@ -2,7 +2,7 @@
 
 import { userPreferencesService } from '@/services/core/user-preferences';
 import { KanbanFilters, ViewPreferences, ColumnVisibility, TaskStatus } from '@/lib/types';
-import { Theme } from '@/lib/theme-config';
+import { Theme } from '@/lib/ui-config';
 import { revalidatePath } from 'next/cache';
 import { getServerSession } from 'next-auth';
 import { authOptions } from '@/lib/auth';