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

webui/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "kiosk-webui",
+  "version": "1.0.0",
+  "main": "server.js",
+  "dependencies": {
+    "express": "^4.19.0"
+  }
+}