Restore completion_date field to TaskCompletionCreateRequest

The completion_date should be provided by the client, not auto-generated
by the backend. Backend has been updated to accept this field.

This allows clients to specify when the task was actually completed,
which may be different from when the completion record is created.

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

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

composeApp/src/commonMain/kotlin/com/example/mycrib/models/TaskCompletion.kt

@@ -42,10 +42,10 @@ data class TaskCompletionCreateRequest(
     @SerialName("completed_by_phone") val completedByPhone: String? = null,
     @SerialName("completed_by_email") val completedByEmail: String? = null,
     @SerialName("company_name") val companyName: String? = null,
+    @SerialName("completion_date") val completionDate: String,
     @SerialName("actual_cost") val actualCost: String? = null,
     val notes: String? = null,
     val rating: Int? = null
-    // Note: completion_date is set automatically by the backend
 )
 
 @Serializable