Add Web UI addon: browser-based config editor (v2.16.0)

New Addons -> Web UI: a small Node/Express app (webui/) installed as a
systemd service running as $KIOSK_USER, giving a browser-based editor
for Sites & Page Timing, Display & Interaction, and Password Protection
& Lockout - the three Core Settings menus that are pure config.json
read/write with no privileged system mutation involved. Runs with no
sudo at all, since config.json is already owned by $KIOSK_USER.

webui/lib/config.js re-implements lib/config.sh's exact schema and
merge-on-save contract in JS (kiosk-app/main.js already reads the same
config.json directly in JS, so this isn't a new pattern), so it can
never silently clobber fields it doesn't track - the same bug
previously fixed in lib/config.sh's own history.

No login of its own by design: Authelia runs elsewhere, and the
expectation is a reverse proxy (e.g. Caddy) with Authelia forward-auth
in front of it, the same way other self-hosted apps get protected -
Authelia integration is explicitly out of scope for this repo.

Deliberately narrow scope for this first pass: WiFi, Timezone,
Power/Display/Quiet Hours, Complete Uninstall, every other addon, and
everything in Advanced remain terminal-only, since a network-facing
process shouldn't be handed sudo-level system mutation without a lot
more thought than this pass gives it. Wired into Complete Uninstall
(webui_do_uninstall) and Clone Settings (addon-presence detection) the
same way every other addon is.

This is the single-kiosk piece of the web-based GUI this repo's
"Modular Management" notes have mentioned for a while - a central
multi-kiosk fleet dashboard is an intentional follow-up.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VfsFSoRqfbRG7XAg5RoE7e

menus/complete_uninstall.sh

@@ -7,7 +7,8 @@
 # (cups_do_uninstall, vnc_do_uninstall, wireguard_do_uninstall,
 # tailscale_do_uninstall, netbird_do_uninstall, lms_do_uninstall,
 # squeezelite_do_uninstall, asterisk_intercom_do_uninstall,
-# power_schedule_do_remove_all, emergency_hotspot_do_disable) instead of
+# webui_do_uninstall, power_schedule_do_remove_all,
+# emergency_hotspot_do_disable) instead of
 # re-implementing removal logic for each addon a second time here - if an
 # addon's uninstall logic changes, this picks it up automatically. Only
 # the pieces no single addon owns - the kiosk user/files, Node.js/
@@ -57,6 +58,7 @@ action_complete_uninstall() {
     echo "  • Squeezelite and LMS (Lyrion Music Server)"
     echo "  • Remote access (VNC, WireGuard, Tailscale, Netbird)"
     echo "  • Asterisk Intercom (Baresip)"
+    echo "  • Web UI"
     echo "  • LightDM and Openbox"
     echo "  • All kiosk schedules and services"
     echo "  • Emergency hotspot configuration"
@@ -86,6 +88,7 @@ action_complete_uninstall() {
     lms_do_uninstall purge
     squeezelite_do_uninstall
     asterisk_intercom_do_uninstall purge
+    webui_do_uninstall
 
     echo "[3/12] Removing schedules and emergency hotspot..."
     power_schedule_do_remove_all