Remove non-functional handlePaste placeholder

[Copilot]

The handlePaste function was a placeholder that only displayed a toast message without implementing actual paste behavior.

Changes:

• Removed handlePaste callback from main-app/index.tsx
• Removed onPaste prop from CustomTitleBar interface and component usage
• Removed unused ClipboardPaste icon import

// Before: Placeholder function that did nothing
const handlePaste = useCallback(() => {
  toast({
    title: "Paste",
    description: "Paste functionality is context-dependent",
    variant: "default",
  });
}, [toast]);

// After: Removed entirely

The Edit menu never referenced this functionality, so removing it has no user-facing impact.

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