Remove REST handlers superseded by GraphQL resolvers

Deleted handle_activity.go, handle_favorites.go, handle_post.go, and
handle_user.go — all logic already exists in schema.resolvers.go.
Removed corresponding REST routes from main.go. Moved UserCredentials
struct (used by Login handler) into handle_auth.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/handlers/handle_auth.go

@@ -10,6 +10,11 @@ import (
 	"golang.org/x/crypto/bcrypt"
 )
 
+type UserCredentials struct {
+	Username string `json:"username" binding:"required"`
+	Password string `json:"password" binding:"required"`
+}
+
 func (store *Store) AuthMiddlewear(ctx *gin.Context) {
 	access_token, err := ctx.Cookie("access_token")
 	if err != nil {