feat: add export/import buttons to mode selector popover

[roomote]

This PR adds export/import functionality directly to the mode selector popover, allowing users to quickly manage modes without navigating away from their current context.

Changes

• Added export button for each mode in the popover list (appears on hover)
• Added import button in the popover header next to existing buttons
• Implemented export/import handlers with loading states
• Added inline import dialog for selecting global/project level
• Reused existing backend message handlers for export/import functionality

Implementation Details

The implementation follows the same pattern as the existing export/import functionality in ModesView.tsx:

• State management for loading indicators (isExporting, isImporting, showImportDialog)
• Message handlers that communicate with the backend via vscode.postMessage
• Inline import dialog that allows users to choose between global and project-level imports

UI/UX Considerations

• Export buttons are hidden by default and appear on hover to avoid cluttering the popover
• Loading states provide visual feedback during export/import operations
• The import dialog matches the design from ModesView for consistency

Testing

• Manually tested export functionality for multiple modes
• Verified import dialog appears and handles both global/project selections
• Confirmed loading states work correctly during operations
• Checked that the popover remains functional with the new buttons

Fixes #6320

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Important

Adds export/import buttons to ModeSelector.tsx popover, enabling quick mode management with new state handling and UI elements.

• UI Changes:
  • Added export button for each mode in ModeSelector.tsx, visible on hover.
  • Added import button in the popover header.
  • Inline import dialog for selecting global/project level.
• State Management:
  • Added isExporting, isImporting, showImportDialog states in ModeSelector.tsx.
• Functionality:
  • Implemented handleExportMode and handleImportMode functions in ModeSelector.tsx.
  • Reused existing backend message handlers for export/import.
• Testing:
  • Manually tested export/import functionality and loading states.

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