Add default timeout control to quiz header with batch apply

[Copilot]

Question timeouts are currently managed individually per question. This adds a default timeout field to the quiz header that can be applied to all questions at once.

Changes

• Model: Added optional DefaultTimeout property to QuestionSet (nullable for backward compatibility)
• UI: New quiz header section with timeout input (HTML min validation) and "Apply to All Questions" button with toast feedback
• Logic: Batch update method validates input and applies timeout to all questions

// New property on QuestionSet
[JsonPropertyName("defaultTimeout")]
public int? DefaultTimeout { get; set; }

// UI interaction
private void ApplyDefaultTimeoutToAllQuestions()
{
    if (_questionSet?.DefaultTimeout > 0)
    {
        foreach (var question in _questions)
            question.Timeout = _questionSet.DefaultTimeout.Value;
        toastService.ShowSuccess($"Applied timeout of {_questionSet.DefaultTimeout.Value}s to {_questions.Count} questions.");
    }
    else
        toastService.ShowError("Please enter a valid timeout value (greater than 0).");
}

Individual question timeouts remain independently editable.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ability to change timeout for all questions at once (eg. default value, or override all)</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

@fortunkam The question timeouts are specific to each question, allow them to be changed in the quiz header.

• Fixes Ability to change timeout for all questions at once (eg. default value, or override all) #25

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