[ux] Ctrl+P / Ctrl+N not mapped for input history navigation; no /terminal command

## Problem

Standard terminal keybinds `Ctrl+P` (previous history) and `Ctrl+N` (next history) are not mapped in the TUI input. Only `Up` / `Down` arrow keys work for history navigation. There is also no `/terminal` command to configure terminal behavior or keybind presets.

## Expected behavior

- `Ctrl+P` should navigate to the previous input history entry (same as `Up` arrow)
- `Ctrl+N` should navigate to the next input history entry (same as `Down` arrow)
- These are readline/emacs-standard keybindings that terminal-native users expect by default

## Current behavior

- `Ctrl+P` and `Ctrl+N` do nothing
- Only `Up` / `Down` arrows work for history (documented in [keybinds page](https://docs.altimate.sh/configure/keybinds/))
- The keybinds are customizable via config, but `Ctrl+P` / `Ctrl+N` should be defaults — not something users have to discover and manually configure

## Additional context

- The [keybinds docs](https://docs.altimate.sh/configure/keybinds/) list 85+ configurable keybindings but omit `Ctrl+P` / `Ctrl+N` from defaults
- No `/terminal` command exists in the [built-in commands](https://docs.altimate.sh/configure/commands/) to adjust terminal behavior presets (e.g., emacs-style vs vi-style input)