feat(auth): implement user authentication and profile management

- Added authentication context and provider to manage user state.
- Created login and registration forms with validation and error handling.
- Implemented user profile component for updating user information and changing passwords.
- Introduced user settings in the database for notification preferences.
- Updated translations for authentication-related strings in English and German.
- Enhanced styles for authentication components and user profile.
- Added middleware for optional and required authentication checks.

backend/src/types/fastify.d.ts

@@ -1,4 +1,12 @@
 import "fastify";
+import "@fastify/jwt";
+
+// User type for authenticated requests
+export interface AuthUser {
+  id: number;
+  username: string;
+  role: string;
+}
 
 declare module "fastify" {
   interface FastifyInstance {
@@ -11,4 +19,16 @@ declare module "fastify" {
       refreshCookieOptions: import("@fastify/cookie").CookieSerializeOptions;
     };
   }
+  
+  interface FastifyRequest {
+    user?: AuthUser | null;
+  }
+}
+
+declare module "@fastify/jwt" {
+  interface FastifyJWT {
+    // Allow flexible payload for access and refresh tokens
+    payload: Record<string, unknown>;
+    user: Record<string, unknown>;
+  }
 }