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.

.gitea/workflows/release.yaml

@@ -55,13 +55,6 @@ jobs:
       - name: Build
         run: npm run build
 
-      - name: Upload build artifacts
-        uses: actions/upload-artifact@v3
-        with:
-          name: ui-build
-          path: ui/build
-          retention-days: 1
-
   # ── docker: build + push all images via docker bake ──────────────────────────
   docker:
     name: Docker
@@ -86,17 +79,6 @@ jobs:
           echo "version=$VER" >> "$GITHUB_OUTPUT"
           echo "major_minor=$(echo "$VER" | cut -d. -f1-2)" >> "$GITHUB_OUTPUT"
 
-      - name: Download ui build artifacts
-        uses: actions/download-artifact@v3
-        with:
-          name: ui-build
-          path: ui/build
-
-      - name: Allow build/ into Docker context (override .dockerignore)
-        run: |
-          grep -v '^build$' ui/.dockerignore > ui/.dockerignore.tmp
-          mv ui/.dockerignore.tmp ui/.dockerignore
-
       - name: Build and push all images
         uses: docker/bake-action@v6
         with: