Fix preview coordinate debug display updates

[JeremyZeng77]

Addresses #24

Summary

• keep the Debug panel's preview position in React state instead of reading the mutable TerrainBuilder ref directly during render
• update preview and camera position together on animation frames so the coordinate display reflects the latest cursor preview position before any wall placement happens
• keep coordinate formatting guarded against missing/non-finite values

Verification

• git diff --check
• npm run build (passes; existing repository warnings remain, including a missing @mediapipe source map and pre-existing lint warnings outside this change)

Bounty / payout

This issue is not currently labeled BOUNTY, but the repository README describes official paid bug/feature bounties via PayPal for accepted bounty PRs. If this fix qualifies, payout can be sent to PayPal: https://paypal.me/Jeremytsangchina

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