Frontend: Update Visual Language of the Visualization Tab
## Goal
Update the visual styling of the Pipeline Editor's Visualization tab to align with the latest Figma designs, without changing interaction behavior.
## Scope
This iteration focuses **only** on visual language updates:
- Updated node styling (colors, borders, typography)
- Updated connection lines and arrows
- Alignment with design system specifications
**Out of scope for this iteration:**
- Drag & drop functionality
- Artifact management
- Other features from the original Visual Canvas proposal
## Acceptance Criteria
- [ ] Visual language matches the latest Figma designs
- [ ] Existing interaction behavior remains unchanged
- [ ] Known limitations (e.g., maximum displayable nodes) are documented for users and stakeholders
## Design Reference
- [Figma](https://www.figma.com/design/pbWiw8cizNEZMOz1ObAVd5/Ideation--AI-first-pipeline-editing-experience?node-id=1607-99138&m=dev)
## Technical implementation
- Update the current graph in: https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/assets/javascripts/ci/pipeline_editor/components/graph
- Use a feature flag to ensure that the previous visualization remains available until full rollout
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