Make the console (kid) account name configurable

Autostart reads CONSOLE_KID_USER from /etc/bc250-console.conf
(default: kid), so renaming the account is a pure on-box operation —
documented in OPERATIONS.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

files/console/usr/share/doc/bc250-console/OPERATIONS.md

@@ -53,6 +53,22 @@ cat /sys/class/drm/card*/device/pp_dpm_sclk   # GPU clock scaling
 df -h /                                       # disk space
 ```
 
+## Renaming the kid's account (optional)
+
+With the kid fully logged out (`sudo loginctl terminate-user kid`):
+
+```bash
+sudo usermod  -l NEWNAME kid
+sudo groupmod -n NEWNAME kid
+sudo usermod  -d /home/NEWNAME -m NEWNAME
+sudo chfn -f "Display Name" NEWNAME
+sudo passwd NEWNAME
+echo 'CONSOLE_KID_USER=NEWNAME' | sudo tee /etc/bc250-console.conf
+```
+
+Steam data and apps live in the home directory and move with it. The last
+line tells the Big Picture autostart which account is the console account.
+
 ## Tuning the GPU (optional, after thermal testing)
 
 The GPU governor's curve lives at