fix: add tmux mouse setup to INSTALL.md, clarify Enter-to-submit on input widgets

INSTALL.md

@@ -127,7 +127,22 @@ Based on what the user chose:
    echo "allow_remote_control yes" >> ~/.config/kitty/kitty.conf
    ```
 
-3. **Optional: install chafa** for ASCII art image fallback:
+3. **If tmux backend chosen**, enable mouse support so the user can click widgets in the TUI pane:
+   ```bash
+   # Check current setting
+   tmux show -g mouse 2>/dev/null
+   ```
+   If mouse is off or not set:
+   ```bash
+   tmux set -g mouse on
+   ```
+   To make it permanent, add to `~/.tmux.conf`:
+   ```bash
+   grep -q "set -g mouse on" ~/.tmux.conf 2>/dev/null || echo "set -g mouse on" >> ~/.tmux.conf
+   ```
+   Without this, tmux intercepts mouse clicks and the user cannot interact with checkboxes, buttons, or inputs in the display pane.
+
+4. **Optional: install chafa** for ASCII art image fallback:
    - Linux: `sudo apt install chafa` or `sudo pacman -S chafa`
    - macOS: `brew install chafa`