configure.sh: seed wizard from setup_system.conf when it exists

Use config/setup_system.conf as the template for configure.conf when
present, so existing values appear as defaults. Falls back to the dist
file on a fresh install.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

system_setup/configure.sh

@@ -5,9 +5,15 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 SCRIPTDIR="$(cd "$(dirname "$0")" && pwd)"
-CONF_DIST="${SCRIPTDIR}/config.dist/setup_system.conf.dist"
 CONF_FILE="${SCRIPTDIR}/../config.d/configure.conf"
 
+CONF_EXISTING="${SCRIPTDIR}/../config/setup_system.conf"
+if [[ -f "$CONF_EXISTING" ]]; then
+    CONF_DIST="$CONF_EXISTING"
+else
+    CONF_DIST="${SCRIPTDIR}/config.dist/setup_system.conf.dist"
+fi
+
 # Prompt for a single value; returns the old value unchanged if the user presses Enter.
 prompt_value() {
     local name="$1" current="$2" new_val