build: drop the Flutter builder stage from the Docker image

The Flutter SDK image is ~2.8 GB and filled Docker Desktop's VM disk
("read-only file system" while extracting a layer). Build the web bundle
on the host instead and COPY client/app/build/web into the 2-stage
(NestJS build -> slim runtime) image. .dockerignore keeps the bundle,
drops the platform scaffolding. README documents the host `flutter build
web` prerequisite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -1,14 +1,11 @@
 # syntax=docker/dockerfile:1
+#
+# The Flutter web bundle is built on the HOST (it needs a ~2.8 GB SDK image
+# otherwise). Before `docker compose build`, run:
+#     (cd client/app && flutter build web --release)
+# This Dockerfile just copies client/app/build/web into the runtime image.
 
-# --- 1. Flutter web build -------------------------------------------------
-FROM ghcr.io/cirruslabs/flutter:stable AS web
-WORKDIR /src
-COPY client/app/pubspec.yaml client/app/pubspec.lock ./
-RUN flutter pub get
-COPY client/app/ ./
-RUN flutter build web --release
-
-# --- 2. Backend build --------------------------------------------------------
+# --- 1. Backend build ---------------------------------------------------------
 FROM node:20-bookworm-slim AS api-build
 WORKDIR /src
 COPY backend/package.json backend/package-lock.json ./
@@ -16,7 +13,7 @@ RUN npm ci
 COPY backend/ ./
 RUN npx prisma generate && npm run build
 
-# --- 3. Runtime ------------------------------------------------------------
+# --- 2. Runtime -------------------------------------------------------------
 FROM node:20-bookworm-slim AS runtime
 ENV NODE_ENV=production
 WORKDIR /app
@@ -26,8 +23,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends openssl \
 COPY --from=api-build /src/node_modules ./node_modules
 COPY --from=api-build /src/dist ./dist
 COPY --from=api-build /src/prisma ./prisma
-# The Flutter web build; app.module reads WEB_CLIENT_DIR.
-COPY --from=web /src/build/web ./web
+# Pre-built Flutter web bundle from the host; app.module reads WEB_CLIENT_DIR.
+COPY client/app/build/web ./web
 ENV WEB_CLIENT_DIR=/app/web
 EXPOSE 3000
 # Apply pending migrations, then boot.