Extract Modal base class and fix target editor defaults

Add core/modal.js with reusable Modal class that handles open/close,
body locking, backdrop close, dirty checking, error display, and a
static stack for ESC key management. Migrate all 13 modals across 8
feature files to use the base class, eliminating ~200 lines of
duplicated boilerplate. Replace manual ESC handler list in app.js
with Modal.closeTopmost(), fixing 3 modals that were previously
unreachable via ESC. Remove 5 unused initialValues variables from
state.js. Fix target editor to auto-select first device/source and
auto-generate name like the KC editor does.

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

server/src/wled_controller/static/js/features/device-discovery.js

@@ -5,13 +5,15 @@
 import {
     _discoveryScanRunning, set_discoveryScanRunning,
     _discoveryCache, set_discoveryCache,
-    settingsInitialValues,
 } from '../core/state.js';
 import { API_BASE, getHeaders, isSerialDevice, escapeHtml, handle401Error } from '../core/api.js';
 import { t } from '../core/i18n.js';
-import { lockBody, unlockBody, showToast } from '../core/ui.js';
+import { showToast } from '../core/ui.js';
+import { Modal } from '../core/modal.js';
 import { _computeMaxFps, _renderFpsHint } from './devices.js';
 
+const addDeviceModal = new Modal('add-device-modal');
+
 export function onDeviceTypeChanged() {
     const deviceType = document.getElementById('device-type').value;
     const urlGroup = document.getElementById('device-url-group');
@@ -154,7 +156,6 @@ export function onSerialPortFocus() {
 }
 
 export function showAddDevice() {
-    const modal = document.getElementById('add-device-modal');
     const form = document.getElementById('add-device-form');
     const error = document.getElementById('add-device-error');
     form.reset();
@@ -172,16 +173,13 @@ export function showAddDevice() {
     document.getElementById('device-serial-port').innerHTML = '';
     const scanBtn = document.getElementById('scan-network-btn');
     if (scanBtn) scanBtn.disabled = false;
-    modal.style.display = 'flex';
-    lockBody();
+    addDeviceModal.open();
     onDeviceTypeChanged();
     setTimeout(() => document.getElementById('device-name').focus(), 100);
 }
 
 export function closeAddDeviceModal() {
-    const modal = document.getElementById('add-device-modal');
-    modal.style.display = 'none';
-    unlockBody();
+    addDeviceModal.forceClose();
 }
 
 export async function scanForDevices(forceType) {