feat(web): improve UI responsiveness, polish, and update docs

- Add mobile/tablet responsive breakpoints to web UI
- Redesign cards as full-bleed poster layout with gradient overlay
- Add skeleton loading state, comic count badge, and search icon
- Switch to Docker image format for registry compatibility
- Add docker-build and docker-push Makefile targets with versioned tags
- Update README to document web UI, Docker deployment, and serve command

Dockerfile

@@ -18,7 +18,7 @@ FROM gcr.io/distroless/base-debian12:nonroot
 
 LABEL org.opencontainers.image.title="yoink" \
       org.opencontainers.image.description="Comic downloader web UI" \
-      org.opencontainers.image.source="https://github.com/bryanlundberg/yoink-go"
+      org.opencontainers.image.source="https://git.brizzle.dev/bryan/yoink-go"
 
 WORKDIR /app