Remove unused components and models for improved maintainability

- Deleted redundant components (`AddActionButton`, `AlertBox`, `AlertStack`, `BackButton`, `CancelButton`, and `CollapsableArea`) and related files.
- Removed unused models (`Book`, `BookSerie`, `BookTables`, `Character`, and `Chapter`) to reduce codebase clutter.
- Updated project structure and references to reflect these removals.

context/LangContext.ts

@@ -2,6 +2,10 @@ import {Context, createContext, Dispatch, SetStateAction} from "react";
 
 export type SupportedLocale = 'fr' | 'en';
 
+export function isSupportedLocale(value: string | null): value is SupportedLocale {
+    return value === 'fr' || value === 'en';
+}
+
 export interface LangContextProps {
     lang: SupportedLocale;
     setLang: Dispatch<SetStateAction<SupportedLocale>>;