Enable CTRL+N and CTRL+P for Navigation Between Slash Commands, File Selector, and History

Improve keyboard accessibility by enabling CTRL+N and CTRL+P to navigate through:

• Slash command options
• File selector dropdowns
• Input history (e.g., previously entered commands or messages)

This brings familiar keybindings from terminal environments (like Vim or Bash) to the app interface, improving efficiency for keyboard-centric users.

Why:
Power users expect CTRL+N / CTRL+P to behave like arrow keys:

• CTRL+N = "next" (down)
• CTRL+P = "previous" (up)

Adding support for input history navigation makes it easier to reuse recent commands or messages.

Acceptance Criteria:

• [ ] CTRL+N and CTRL+P navigate through slash command suggestions.
• [ ] CTRL+N and CTRL+P navigate through file selector options.
• [ ] CTRL+N and CTRL+P cycle through input history when slash/file menu is not open.

Labels:
enhancement accessibility keyboard-navigation productivity