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/FirstGroupStep.tsx

@@ -89,8 +89,8 @@ export function FirstGroupStep({ onBack, onCompleted }: FirstGroupStepProps) {
                 className={cn(
                   'h-9 rounded-full border px-3.5 text-sm font-medium transition-colors',
                   isActive
-                    ? 'border-accent bg-accent text-white'
-                    : 'border-border bg-white text-primary hover:bg-primary-soft',
+                    ? 'border-accent bg-accent text-on-accent'
+                    : 'border-border bg-surface text-primary hover:bg-primary-soft',
                 )}
               >
                 {label}
@@ -168,7 +168,7 @@ export function FirstGroupStep({ onBack, onCompleted }: FirstGroupStepProps) {
                   'rounded-xl border px-3 py-2.5 text-left transition-colors',
                   isActive
                     ? 'border-accent bg-accent-soft'
-                    : 'border-border bg-white hover:bg-primary-soft',
+                    : 'border-border bg-surface hover:bg-primary-soft',
                 )}
               >
                 <span