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/kc-targets.ts

@@ -195,9 +195,9 @@ export function createKCTargetCard(target: OutputTarget & { state?: any; metrics
     const brightness = kcSettings.brightness ?? 1.0;
     const brightnessInt = Math.round(brightness * 255);
 
-    const source = sourceMap[target.picture_source_id];
+    const source = sourceMap[target.picture_source_id!];
     const sourceName = source ? source.name : (target.picture_source_id || 'No source');
-    const patTmpl = patternTemplateMap[kcSettings.pattern_template_id];
+    const patTmpl = patternTemplateMap[kcSettings.pattern_template_id!];
     const patternName = patTmpl ? patTmpl.name : 'No pattern';
     const rectCount = patTmpl ? (patTmpl.rectangles || []).length : 0;
 
@@ -226,7 +226,7 @@ export function createKCTargetCard(target: OutputTarget & { state?: any; metrics
         content: `
             <div class="card-header">
                 <div class="card-title" title="${escapeHtml(target.name)}">
-                    ${escapeHtml(target.name)}
+                    <span class="card-title-text">${escapeHtml(target.name)}</span>
                 </div>
             </div>
             <div class="stream-card-props">
@@ -617,7 +617,7 @@ export async function showKCEditor(targetId: any = null, cloneData: any = null)
         // Tags
         if (_kcTagsInput) _kcTagsInput.destroy();
         _kcTagsInput = new TagInput(document.getElementById('kc-tags-container'), {
-            placeholder: window.t ? t('tags.placeholder') : 'Add tag...'
+            placeholder: t('tags.placeholder'),
         });
         _kcTagsInput.setValue(_editorTags);