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/routes/onboarding.tsx

@@ -63,7 +63,7 @@ export function OnboardingView() {
       <div className="flex min-h-dvh items-center justify-center px-4">
         <div className="w-full max-w-sm space-y-4 text-center">
           <span className="mx-auto flex size-14 items-center justify-center rounded-2xl bg-accent-soft">
-            <LogoIcon className="size-8 text-accent" />
+            <LogoIcon className="size-8" />
           </span>
           <h1 className="text-2xl font-bold text-primary">Tu cuenta, lista</h1>
           <p className="text-sm text-foreground/60">
@@ -92,9 +92,8 @@ export function OnboardingView() {
 
   return (
     <main className="mx-auto flex min-h-dvh w-full max-w-md flex-col px-4 pb-12 pt-8 sm:pt-12">
-      <header className="mb-6 flex items-center justify-center gap-2">
-        <LogoIcon className="size-7" />
-        <Logo className="text-xl" />
+      <header className="mb-6 flex items-center justify-center">
+        <Logo className="h-6" />
       </header>
 
       <Stepper steps={STEPS} current={step} />