Edit mode with emacs keybindings

[rodionsteshenko]

Add the ability to edit files directly in the diff view, using emacs-style keybindings.

Desired behavior:

• Press e to enter edit mode on the focused pane
• Emacs keybindings: C-a (start of line), C-e (end of line), C-k (kill line), C-y (yank), C-w (cut region), M-w (copy region)
• Visual indicator showing edit mode is active
• Save with C-x C-s, cancel with C-g
• Diff updates live as you edit

This is the most requested feature for the TUI version.