fix: resolve all TypeScript strict null check errors

Fix ~68 pre-existing strict null errors across 13 feature modules.
Add non-null assertions for DOM element lookups, null coalescing for
optional values, and type guards for nullable properties. Zero tsc
errors now with --noEmit.

server/src/wled_controller/static/js/features/devices.ts

@@ -38,7 +38,7 @@ function _ensureSettingsCsptSelect() {
                 icon: ICON_TEMPLATE,
                 desc: '',
             })),
-            placeholder: window.t ? t('palette.search') : 'Search...',
+            placeholder: t('palette.search'),
             allowNone: true,
             noneLabel: t('common.none_no_cspt'),
         } as any);
@@ -443,7 +443,7 @@ export async function showSettings(deviceId: any) {
         // Tags
         if (_deviceTagsInput) _deviceTagsInput.destroy();
         _deviceTagsInput = new TagInput(document.getElementById('device-tags-container'), {
-            placeholder: window.t ? t('tags.placeholder') : 'Add tag...'
+            placeholder: t('tags.placeholder'),
         });
         _deviceTagsInput.setValue(device.tags || []);