feat: Replace Delete Chat button with Retry Generation button

[Copilot]

This PR replaces the "Delete Chat" button with a more useful "Retry Generation" button that allows users to regenerate the last AI response instead of deleting the entire chat session.

Changes Made

UI Updates

• Replaced the red delete icon with an orange clockwise arrow icon (ArrowClockwise)
• Updated button tooltip from "Delete Current Chat (Ctrl+D)" to "Retry Last AI Response (Ctrl+R)"
• Changed keyboard shortcut from Ctrl+D to Ctrl+R

New Retry Functionality

The retry feature intelligently:

• Finds the last user message in the conversation
• Removes any subsequent AI/assistant responses after that message
• Regenerates the AI response using the same user input
• Preserves all conversation history except the retried response
• Maintains compatibility with Mem0 memory integration

Implementation Details

// New retry method in AI service
public static async Task RetryLastResponseAsync(MarkdownScrollViewer outputBox)
{
    // Find last user message and remove subsequent AI responses
    // Regenerate AI response with same input
    // Update UI and save to file
}

Internationalization

Updated all localization files:

• English: "Retry Last AI Response (Ctrl+R)"
• Chinese Simplified: "重新生成 AI 回复 (Ctrl+R)"
• Chinese Traditional: "重新生成 AI 回复 (Ctrl+R)"

Benefits

• Non-destructive: Preserves conversation history unlike the delete button
• User-friendly: Easy way to get better AI responses without retyping
• Efficient: Only regenerates the last response rather than starting over
• Consistent: Follows existing error handling and UI patterns

This change makes the application more user-friendly by replacing a destructive action (delete entire chat) with a constructive one (improve the last response).

Fixes #9.

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