chore(mobile): extract ENDED_STATE_LINGER_MS constant

The 2500ms timer that drifts CallState from `ended` back to `idle` was
an inline magic number. Promote to a module-level constant with a
comment explaining why the value isn't arbitrary — picked from the
post-Phase-3 quality review.

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

apps/mobile/lib/callContext.tsx

@@ -62,6 +62,12 @@ function randomCallId(): string {
   return 'call-' + Math.random().toString(36).slice(2, 10) + '-' + Date.now().toString(36);
 }
 
+// Time the call state lingers in `ended` so the UI can render a transient
+// status pill ("Anruf abgelehnt", "Anruf beendet") before snapping back
+// to idle. Keep this comfortably above the user's reaction time but short
+// enough that returning to a chat feels snappy.
+const ENDED_STATE_LINGER_MS = 2500;
+
 export function CallProvider({ children }: { children: React.ReactNode }) {
   const { user } = useAuth();
   const myUserId = user?.id ?? null;
@@ -353,13 +359,13 @@ export function CallProvider({ children }: { children: React.ReactNode }) {
     }
   }, [state]);
 
-  // Drift `ended` back to `idle` after a short pause so the UI can show
+  // Drift `ended` back to `idle` after the linger window so the UI can
-  // a brief status pill ("Anruf abgelehnt") before snapping back.
+  // show a brief status pill before snapping back.
   useEffect(() => {
     if (state.kind !== 'ended') return;
     const t = setTimeout(() => {
       setState({ kind: 'idle' });
-    }, 2500);
+    }, ENDED_STATE_LINGER_MS);
     return () => clearTimeout(t);
   }, [state]);