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

install.sh

@@ -32,7 +32,12 @@
 #     (menus/addon_authelia.sh), Remote Access - VNC/WireGuard/
 #     Tailscale/Netbird (menus/addon_remote_access.sh), LMS Server /
 #     Squeezelite Player (menus/addon_lms_squeezelite.sh), Asterisk
-#     Intercom - SIP extension client (menus/addon_asterisk_intercom.sh).
+#     Intercom - SIP extension client (menus/addon_asterisk_intercom.sh),
+#     Web UI (menus/addon_webui.sh - browser-based editor for Sites/
+#     Display/Lockout, the webui/ Node app installed as a systemd
+#     service; no login of its own, put it behind your own reverse proxy
+#     with Authelia forward-auth if it needs to be reachable beyond a
+#     trusted LAN - see the file header).
 #   Advanced: Diagnostics (menus/diagnostics.sh - system status/logs/
 #     audio/network), Electron Maintenance (menus/advanced_electron.sh -
 #     manual update, fix blank screen), Factory Reset
@@ -87,6 +92,8 @@ source "$SCRIPT_DIR/menus/addon_remote_access.sh"
 source "$SCRIPT_DIR/menus/addon_lms_squeezelite.sh"
 # shellcheck source=menus/addon_asterisk_intercom.sh
 source "$SCRIPT_DIR/menus/addon_asterisk_intercom.sh"
+# shellcheck source=menus/addon_webui.sh
+source "$SCRIPT_DIR/menus/addon_webui.sh"
 # shellcheck source=menus/advanced_electron.sh
 source "$SCRIPT_DIR/menus/advanced_electron.sh"
 # shellcheck source=menus/advanced_upgrade.sh
@@ -167,8 +174,8 @@ core_settings_menu() {
 }
 
 addons_menu_builder() {
-    MENU_LABELS=("CUPS Printing" "Authelia Auto-Login" "Remote Access" "LMS Server / Squeezelite Player" "Asterisk Intercom (SIP Extension)")
-    MENU_HANDLERS=(addon_cups_menu addon_authelia_menu remote_access_menu addon_lms_squeezelite_menu addon_asterisk_intercom_menu)
+    MENU_LABELS=("CUPS Printing" "Authelia Auto-Login" "Remote Access" "LMS Server / Squeezelite Player" "Asterisk Intercom (SIP Extension)" "Web UI")
+    MENU_HANDLERS=(addon_cups_menu addon_authelia_menu remote_access_menu addon_lms_squeezelite_menu addon_asterisk_intercom_menu addon_webui_menu)
 }
 
 addons_menu() {