Add TUI file selection interface

[sgrimee]

Summary

Create a file selection popup/dialog in the TUI for choosing files to attach to messages.

Requirements

• Popup window for file browser
• Keyboard navigation for file selection
• Integration with message composition flow
• Cancel/confirm file selection

Dependencies

• Requires CLI file sending capability to be working

Acceptance Criteria

• File selection popup opens from message composer
• Users can navigate directories with keyboard
• Selected files integrate with message sending
• Proper error handling for inaccessible files

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