stepped checkout, auth pages, sticky stepper bar, action footer

- 4-step checkout (cart, account, shipping, payment) with Pinia store and route children
- minimal cream sticky top bar (logo + stepper + back-to-shopping) replacing nav/footer in checkout
- fixed cream action footer at viewport bottom for back/continue on every step
- guest / sign-in / register tabs on account step + standalone /login and /register
- src/api/auth.js stub matching expected backend contract
- legal pages (/impressum, /datenschutz) and category landings (/pflege, /haushalt)
- success page stacked CTAs (back-to-home primary, keep-shopping secondary)
- footer logo size aligned with navbar; cream variant footer

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/LegalPage.vue

@@ -96,7 +96,7 @@ onBeforeUnmount(() => {
 <template>
   <Navbar
     ref="navRef"
-    variant="paper"
+    variant="cream"
     layout="standard"
     :items="navItems"
     :cart-count="cart.count"