refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/Dockerfile

@@ -0,0 +1,17 @@
+# Build context: web-player/
+FROM node:22-alpine AS build
+WORKDIR /src
+COPY package.json package-lock.json ./
+RUN npm install
+COPY . .
+RUN npm run build
+
+FROM node:22-alpine AS runtime
+WORKDIR /app
+ENV NODE_ENV=production
+COPY --from=build /src/build ./build
+COPY --from=build /src/node_modules ./node_modules
+COPY --from=build /src/package.json ./package.json
+EXPOSE 3000
+ENV PORT=3000
+CMD ["node", "build/index.js"]