fix: set app user shell to /bin/bash so VSCode terminals work

baseimage-gui creates the app user (UID 1000) at runtime with
shell=/sbin/nologin and home=/dev/null. VSCode tries to spawn the
user's login shell for terminals, which fails with exit code 1.

Adds a cont-init script that runs as root after baseimage-gui's
adduser step and corrects both the shell and home directory.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

scripts/cont-init-user.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# Fix the app user (UID 1000) created by baseimage-gui at runtime.
+# baseimage-gui sets shell=/sbin/nologin and home=/dev/null, which
+# prevents VSCode from opening terminals.
+usermod -s /bin/bash app
+usermod -d /home/user app