fix(mobile/ios): back button showed "< (tabs)" label

iOS labels the back button with the previous screen's title, which was
the untitled (tabs) route group. Use headerBackButtonDisplayMode
"minimal" (chevron only) for all stack screens.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

mobile/app/(app)/_layout.tsx

@@ -31,6 +31,9 @@ export default function AppLayout() {
     // found.md: "kein Zurückbutton" on the health screen wasn't a missing
     // button, it was a black-on-black one).
     headerTintColor: colors.ink,
+    // iOS labels the back button with the previous screen's title — here the
+    // untitled `(tabs)` group, so it literally read "< (tabs)". Chevron only.
+    headerBackButtonDisplayMode: "minimal",
   } as const;
 
   return (