fix: simplify Docker build workflow, remove PREBUILT artifact workaround

- Remove UI build artifact upload/download steps (18 lines removed)
- Remove .dockerignore manipulation workaround
- Always build UI from source inside Docker (more reliable)
- Remove PREBUILT arg from ui/Dockerfile and docker-bake.hcl

This fixes the '/app/build not found' error in CI by eliminating the fragile
artifact-passing mechanism. UI now builds fresh in Docker using build cache,
same as local development.

ui/Dockerfile

@@ -21,11 +21,7 @@ ENV PUBLIC_BUILD_VERSION=$BUILD_VERSION
 ENV PUBLIC_BUILD_COMMIT=$BUILD_COMMIT
 ENV PUBLIC_BUILD_TIME=$BUILD_TIME
 
-# PREBUILT=1 skips npm run build — used in CI when the build/ directory has
-# already been compiled (and debug IDs injected) by a prior job.  The caller
-# must copy the pre-built build/ into the Docker context before building.
-ARG PREBUILT=0
-RUN [ "$PREBUILT" = "1" ] || npm run build
+RUN npm run build
 
 # ── Runtime image ──────────────────────────────────────────────────────────────
 # adapter-node bundles most server-side code, but packages with dynamic