FIX: legend sync, theme, and click handling

[YektaY]

Description

• Legend entries now properly disappear when curves are toggled off and reappear when toggled back on (uses pyqtgraph's removeItem/addItem since QGraphicsGridLayout doesn't collapse hidden items)
• Disabled pyqtgraph's built-in legend click-to-toggle which was changing curve visibility without updating the control panel toggles
• Fixed light mode checkboxes rendering as dark squares (unchecked color was #2A2A2A copied from dark theme)
• Stylesheet asset URLs now resolve to absolute paths for icon loading

Motivation

Addresses a few small issues

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions

[zdomke]

Everything works as expected.

Thank you for making some of these changes. 🙏 They've plagued me for ages. I've had nightmares about these checkboxes.