fix(customer): orders empty CTA opens /book; hide leaked "(tabs)" back title

- the "Book a wash" empty-state action went to home (router.replace('/')); now it
  pushes /book directly, same as home's Book-a-wash CTA.
- header back button was showing the group route name "(tabs)" as its title;
  set headerBackButtonDisplayMode 'minimal' (chevron only) across the stack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Banyel3

apps/customer-mobile/src/app/(tabs)/orders.tsx

@@ -65,7 +65,7 @@ export default function OrdersScreen() {
           title="No orders yet"
           subtitle="Book your first express wash and it'll show up here."
           actionLabel="Book a wash"
-          onAction={() => router.replace('/')}
+          onAction={() => router.push('/book')}
         />
       </Screen>
     );

apps/customer-mobile/src/app/_layout.tsx

@@ -66,6 +66,8 @@ export default function RootLayout() {
             headerTintColor: colors.text,
             headerTitleStyle: { fontFamily: font.bold, color: colors.text },
             headerShadowVisible: false,
+            // Chevron only — hide the previous route's title (was leaking "(tabs)").
+            headerBackButtonDisplayMode: 'minimal',
             contentStyle: { backgroundColor: colors.bg },
           }}
         >