feat: add red theme and board background modal

- Introduced a new red theme with custom color variables for a bold aesthetic.
- Updated the theme management to include the new red theme option.
- Added a BoardBackgroundModal component for managing board background settings, including image uploads and opacity controls.
- Enhanced KanbanCard and KanbanColumn components to support new background settings such as opacity and border visibility.
- Updated API client to handle saving and deleting board backgrounds.
- Refactored theme application logic to accommodate the new preview theme functionality.

apps/app/src/components/views/settings-view/shared/types.ts

@@ -29,7 +29,8 @@ export type Theme =
   | "gruvbox"
   | "catppuccin"
   | "onedark"
-  | "synthwave";
+  | "synthwave"
+  | "red";
 
 export type KanbanDetailLevel = "minimal" | "standard" | "detailed";