fix: update Dockerfile to correct paths for package.json and source files

frontend/Dockerfile

@@ -1,9 +1,9 @@
 # Etap 1: Build Vue
 FROM node:20 AS build
 WORKDIR /app
-COPY frontend/package*.json ./
+COPY ./package*.json ./
 RUN npm install
-COPY frontend ./
+COPY . ./
 RUN npm run build
 
 # Etap 2: Nginx serwujący Vue + proxy do backendu