Rebrands the Music Lab pack selection dialog missed in the earlier Music Lab pass.

• Uses MUI Typography for dialog and row text.
• Moves modal colors from legacy/raw values to semantic dark-theme tokens.
• Keeps DSCO SegmentedButtons because the MUI migration status says segmented buttons are not started.
• Sets the dialog subtree to data-theme="Dark" so the semantic tokens resolve to the dark Music Lab surface.

Testing story

Before entire modal:

After entire modal:

Before select song:

After select song:

Manual QA checklist before marking ready:

• Open a Music Lab level with no selected pack under ?brand=codeai-next.
• Confirm the Select a track modal uses the rebrand dark surface, border, typography, and button styling.
• Switch Popular, Song A-Z, and Artist A-Z modes.
• Select a row and confirm the selected row state has readable text and the Select button behavior still works.
• Verify Skip, Select, Escape, tab order, and focus trap behavior.
• Check desktop widths around 1568, 1280, 1024, and 768 px.
• Check Firefox or Safari if available.
• Check RTL if the test environment makes that easy.

Design check:

• The old alternate row color was raw #363E48. There is no exact semantic dark token for it, so this uses --background-neutral-secondary, which makes the alternating rows stronger.
• The selected row now uses --background-brand-teal-primary; row text switches to --text-neutral-white-fixed so selected normal-size text passes AA contrast.