Refresh topbar header image

[danoli3]

Summary

• Replace the static topbar.png raster with a generated SVG flow-field pattern (same generative-art spirit, but vector so it stays crisp and scales cleanly instead of tiling/cropping at odd window widths)
• background-size: cover + no-repeat on .topBar so it always fills the header regardless of window width

Test plan

• Launched the Electron GUI, screenshotted the header, confirmed it renders correctly
• No console/page errors on load