feat: add deletion confirmation dialog

[struong]

motivation

Accidental repository deletion is irreversible and can cause significant data loss. Users need a safety mechanism to prevent unintended deletions when pressing Enter in the staging area.

change

• Added ConfirmDeletion app mode between Staging and Deleting
• Created a centered confirmation dialog that prompts users to type the number of repos to delete
• Modified Enter key in Staging mode to show confirmation dialog instead of immediately deleting
• Added confirmation_input field to AppState for tracking user input
• Updated header to show "CONFIRM" mode indicator
• Added keybindings for confirmation mode (Enter to confirm, Esc to cancel)

testing

• cargo build and cargo clippy pass
• Manual testing:
  1. Stage repos for deletion → Press Enter
  2. Dialog appears: "Type X to delete X repos:"
  3. Type correct number + Enter → Deletion proceeds
  4. Type wrong number + Enter → Warning message shown
  5. Press Esc → Returns to Staging mode