docs: replace preview screenshot with hand-drawn SVG illustration

[mateuseap]

Summary

• Replace the README's screenshot (docs/assets/preview.png) with a custom hand-authored SVG illustration of PixelHub's actual game world (docs/assets/preview.svg), so the preview never goes stale when the UI changes.
• Illustrates the real client palette from client/src/game/textures.ts: dark void framing, plum wall with back-wall face/edge shading, wood floor tiles with speckle grain, a red rug and a teal rug, two potted plants, and three round-headed avatars, one showing the green proximity-voice ring and a speech bubble.
• Tighten the gap between the badge row and the preview image by removing the leading <br /> (the trailing <br /> before the --- divider is unchanged).
• Remove docs/assets/preview.png (confirmed via grep it was referenced nowhere else in the repo).

Test plan

• Open the PR preview / pull the branch and confirm the SVG renders correctly in the GitHub README
• Confirm the image displays at width=720 with no layout shift versus the old screenshot