fix: harden whiteboard feedback reliability

[Jeremygarden]

Summary

• Replace spread-based whiteboard text measurement with reduce-based sizing to avoid crashes on edge-case multiline labels.
• Validate PNG preview data by signature before writing whiteboard feedback sidecars.
• Add regression tests for empty multiline labels and fake PNG data URLs.

Tests

• npm test -- test/whiteboard-core.test.js test/whiteboard-store.test.js test/server-whiteboard.test.js
• npm run build
• npm run lint
• npm run format:check

Note: full npm test still has local baseline failures unrelated to this branch: Cursor registration behavior in this environment, and a browser rendering assertion reproduced on main.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The text-measurement changes preserve minimum dimensions while removing spread-related failure modes, and the PNG check strictly narrows previously accepted preview inputs.

_{Reviews (1): Last reviewed commit: "fix: harden whiteboard feedback reliabil..." | Re-trigger Greptile}

[kunchenguid]

Speaking as Kun's firstmate.

This is waiting on you, not on a captain decision.

Fork diff reviewed (head 5edd3339): whiteboard empty-line Math.max(...[]) crash fix, PNG signature check before sidecar write, tests. No workflow/deps. Not a security flag. Corrective. Approved CI, Guard, and Require no-mistakes so those runs can start.

Require no-mistakes is blocking, and the PR body has no pipeline attestation for 5edd3339. After a matching raise on that head, we'll look again.

VISION: all seven align (whiteboard measurement/preview hardening; Mermaid source stays authoritative; no default-behavior change).