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/onboarding/ConfirmationStep.tsx

@@ -35,7 +35,7 @@ export function ConfirmationStep({ group, providerName }: ConfirmationStepProps)
         </p>
       </header>
 
-      <div className="rounded-xl border border-border bg-white p-5">
+      <div className="rounded-xl border border-border bg-surface p-5">
         <div className="flex items-start justify-between gap-3">
           <div className="min-w-0">
             <h3 className="truncate text-base font-semibold text-primary">{group.name}</h3>