Fix iOS build errors - Update TaskDetail model usage for Double cost fields

Fixed type conversion issues in iOS Swift files to handle KotlinDouble for
cost fields instead of String. Updated all task-related views to properly
convert between String (for TextField input) and KotlinDouble (for API calls).

Changes:
- TaskCard.swift: Updated preview data with new TaskDetail signature (added
  residenceName, createdBy, createdByUsername, intervalDays; changed
  estimatedCost from String to Double; removed actualCost and notes)
- TasksSection.swift: Updated two preview TaskDetail instances with new signature
- CompleteTaskView.swift: Convert actualCost String to KotlinDouble for API
- EditTaskView.swift: Convert estimatedCost between KotlinDouble and String
  for display and API calls
- TaskFormView.swift: Convert estimatedCost String to KotlinDouble for API

Pattern used:
- Display: KotlinDouble -> String using .doubleValue
- API: String -> Double -> KotlinDouble using KotlinDouble(double:)

Build now succeeds with all type conversions properly handling Decimal/Double
values from backend instead of String.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

iosApp/iosApp/Task/TaskFormView.swift

@@ -318,7 +318,7 @@ struct TaskFormView: View {
             priority: Int32(priority.id),
             status: selectedStatus.map { KotlinInt(value: $0.id) },
             dueDate: dueDateString,
-            estimatedCost: estimatedCost.isEmpty ? nil : estimatedCost,
+            estimatedCost: estimatedCost.isEmpty ? nil : KotlinDouble(double: Double(estimatedCost) ?? 0.0),
             archived: false
         )