feat(auth): implement JWT authentication flow

This commit implements a full-stack authentication system using JWT.

On the backend, a new login endpoint `/api/token` is created. It
validates credentials and issues a signed JWT. A JWT middleware is added
to the server configuration to protect sensitive API endpoints like
`/api/category` and `/api/operation`. Configuration is now managed via a
`wallet.conf` file, loaded at startup.

On the frontend, this enables a complete authentication lifecycle: - A
new `/login` page allows users to enter their credentials. - A
`PrivateRoute` component wraps protected pages, redirecting
unauthenticated users to the login page. - API calls to protected
resources now include the `Authorization: Bearer <token>` header. - A
logout function is added to clear user session data.

refactor(ui): enhance history and category views

This commit also introduces major UI improvements.

The History modal has been completely redesigned. Operations are no
longer displayed as a flat list but are now grouped into tabs for Daily,
Weekly, Monthly, and Yearly views, providing a much clearer overview of
transactions.

The Categories modal now separates categories into "Incomes" and
"Expenses" tabs, reflecting the changes in the backend data model.

backend/go.mod

@@ -3,15 +3,17 @@ module backend
 go 1.24.5
 
 require (
-	github.com/gofiber/fiber v1.14.6
+	github.com/gofiber/contrib/jwt v1.1.2
 	github.com/gofiber/fiber/v2 v2.52.9
+	github.com/golang-jwt/jwt v3.2.2+incompatible
+	github.com/joho/godotenv v1.5.1
 )
 
 require (
+	github.com/MicahParks/keyfunc/v2 v2.1.0 // indirect
 	github.com/andybalholm/brotli v1.1.0 // indirect
-	github.com/gofiber/utils v0.0.10 // indirect
+	github.com/golang-jwt/jwt/v5 v5.2.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
-	github.com/gorilla/schema v1.1.0 // indirect
 	github.com/klauspost/compress v1.17.9 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect