feat: add desktop notification when agent prompts with multichoice question

[1code-async]

Summary

• Adds a native OS notification when the Claude agent uses the AskUserQuestion tool (multichoice prompts)
• Notification only appears when the app window is not focused, helping users know when the agent needs input even if they've switched to another application
• Uses existing notifyAgentNeedsInput() function which respects user notification settings and includes priority-based throttling

Implementation

Added a useEffect in ChatViewInner that watches for changes to pendingQuestions (from pendingUserQuestionsAtom). When a new question arrives (tracked by toolUseId), it triggers notifyAgentNeedsInput() which:

• Checks if window is focused (doesn't notify if user is already looking at the app)
• Respects the user's notification preferences
• Uses priority-based throttling to prevent notification spam

Test plan

• Open a chat and start an agent task that will ask a question
• Switch to another application while the agent is working
• Verify a notification appears when the agent asks a multichoice question
• Verify no notification appears if you stay focused on the app
• Verify notifications respect the user's notification settings

Fixes: ENG-586

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