Switch frontend from npm to pnpm

- Dockerfile: uses corepack + pnpm with --frozen-lockfile
- Lock file: pnpm-lock.yaml replaces package-lock.json
- Build verified: all routes compile OK

frontend/Dockerfile

@@ -1,10 +1,11 @@
 FROM node:24-alpine AS base
+RUN corepack enable && corepack prepare pnpm@latest --activate
 
 # --- DEPENDENCIES ---
 FROM base AS deps
 WORKDIR /app
-COPY package.json package-lock.json* ./
-RUN npm ci
+COPY package.json pnpm-lock.yaml ./
+RUN pnpm install --frozen-lockfile
 
 # --- BUILD ---
 FROM base AS builder
@@ -14,7 +15,7 @@ COPY . .
 
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
-RUN npm run build
+RUN pnpm build
 
 # --- PRODUCTION ---
 FROM base AS runner