Preserve checkbox inline styles when exporting block PNG/SVG

[tracygardner]

Motivation

• Export was replacing entire inline style attributes on text nodes which clobbered checkbox-specific display/visibility styles and caused exported images to misrepresent checked/unchecked boxes.
• The goal is to normalize text appearance for readability while preserving checkbox inline behavior and any pre-existing inline style properties that should not be overwritten.

Description

• In generateSVG(block) (main/export.js) changed the selector to target text.blocklyText and switched from setAttribute("style", ...) to per-property updates via textElement.style.fill, textElement.style.stroke, and textElement.style.fontWeight.
• Detect .blocklyCheckbox on text nodes and skip applying readability overrides (stroke and fontWeight) to checkbox text nodes while still enforcing a normalized fill color.
• Kept existing embedding of fonts and the stylesheet injection intact so exported SVGs still use the project font and field rect background normalization.

Testing

• Ran a production build with npm run build which completed successfully.
• Attempted an automated headless validation that exercised the block export and captured the generated SVG via Playwright, but the export callback environment did not expose the needed registry hooks in the headless run so checkbox-visibility confirmation was only partially validated (script ran but could not fully capture the export output).

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Codex Task