feat: use VS Code hover widget colors for tooltip backgrounds

[lukecotter]

📝 PR Overview

Switch timeline and data grid tooltips from generic editor/widget background colors to the more specific editorHoverWidget theme variables, improving visual consistency with VS Code's native hover styling.

🛠️ Changes made

• Add --tl-hover-background, --tl-hover-border, --tl-hover-foreground CSS custom properties mapped to editorHoverWidget theme colors with editorWidget fallbacks
• Update timeline tooltip styles (tooltip-utils.ts, timeline.css.ts) to use the new hover-specific variables
• Update data grid tooltip styles (DataGrid.scss) to use editorHoverWidget colors and move .tabulator-tooltip to top-level scope for correct specificity
• Move color and opacity styling from .tooltip-value to .tooltip-label for better visual hierarchy in timeline tooltips

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

✅ Tests added?

• 👍 yes
• 🙅 no, not needed

📚 Docs updated?

• 🙅 not needed