UI: Refine color scheme with custom palette and dark mode

[nick-transition]

Summary

Move from the single teal seed color to a richer custom ColorScheme with a warm accent color, and add dark mode support.

Details

• Define a custom ColorScheme with deeper teal/emerald primary and a warm accent (coral or amber) for CTAs
• Add dark mode ColorScheme variant
• Use a toggle or system preference to switch themes
• Leverage M3 surface tint and container colors for depth

Priority

Medium impact, low effort