fix: Improved legend vertical alignment

[ClaudioGSDB]

Description

Legend markers weren't vertically centered against their labels, the marker sat a bit high/low
depending on the font.

The marker used a fixed 2px top margin to fake the alignment, which only worked for one font's
metrics. Swapped that for real centering: align-items: center on the legend item, and flex-center the
marker's SVG (also kills the inline-SVG baseline gap). Now it's pure geometry, so it lines up
regardless of font/theme.

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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