Harden iOS app with audit fixes, UI consistency, and sheet race condition fixes

Applies verified fixes from deep audit (concurrency, performance, security,
accessibility), standardizes CRUD form buttons to Add/Save pattern, removes
.drawingGroup() that broke search bar TextFields, and converts vulnerable
.sheet(isPresented:) + if-let patterns to safe presentation to prevent
blank white modals.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

iosApp/iosApp/Task/CompletionHistorySheet.swift

@@ -262,7 +262,7 @@ struct CompletionHistoryCard: View {
                             .foregroundColor(Color.appPrimary)
                     }
 
-                    Text("$\(cost)")
+                    Text(cost.toCurrency())
                         .font(.system(size: 15, weight: .bold, design: .rounded))
                         .foregroundColor(Color.appPrimary)
                 }