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>

.dockerignore

@@ -1,10 +1,17 @@
 **/node_modules
-**/dist
-**/build
 **/.dart_tool
 **/coverage
-.git
 **/*.log
+.git
+.github
+backend/dist
+# Flutter platform scaffolding / caches — the build/web bundle IS needed.
+client/app/android
+client/app/ios
+client/app/linux
+client/app/macos
+client/app/windows
+client/app/.dart_tool
 # Secrets: passed at runtime via env_file / bind mount, never baked in.
 backend/.env
 backend/serviceAccount.json