feat: add brand assets, design tokens and light/dark theme

- Use real logo/isotype SVGs across app and emails
- Token-based color system aligned with landing (brand scale + semantic tokens)
- ThemeProvider (light/dark/system) with header toggle and settings card
- Embed logo in transactional emails; centralize email colors

apps/web/src/components/ui/avatar.tsx

@@ -22,7 +22,7 @@ export const Avatar = forwardRef<HTMLDivElement, AvatarProps>(
       <div
         ref={ref}
         className={cn(
-          'flex size-9 shrink-0 items-center justify-center overflow-hidden rounded-xl bg-accent text-sm font-semibold text-white',
+          'flex size-9 shrink-0 items-center justify-center overflow-hidden rounded-xl bg-accent text-sm font-semibold text-on-accent',
           className,
         )}
         {...props}