Fix thumbnail data URL MIME type detection

[remcoder]

Problem

PNG thumbnails such as those embedded in demo/gcodes/screw.gcode declare image/jpeg in their data URLs, giving consumers the wrong content type. Fixes #504.

Cause

Thumbnail.src hardcodes the JPEG prefix regardless of the embedded image format. Browsers can sniff the payload, masking the mismatch.

Fix

Decode only the leading base64 characters and identify PNG, JPEG, or QOI from their magic bytes. Keep the existing JPEG fallback for unknown or malformed data so accessing src remains tolerant of incomplete input.

Behavior changes

PNG and QOI payloads now declare image/png and image/qoi; JPEG retains image/jpeg. The payload and getter signature remain unchanged.

Checks

• npm run test:coverage: 984 tests pass; 100% statement, branch, function, and line coverage.
• npm run typeCheck
• npm run lint
• npm run build
• Regression coverage for PNG parsing, JPEG, QOI, unknown, empty, short, and malformed payloads.
• Labelled bug and parser.

Assisted by Codex - GPT-6

[github-actions]

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