Replace flat sub-tab bars with tree sidebar navigation

Add TreeNav component that groups related entity types into a
collapsible hierarchy for Targets and Sources tabs. Targets tree
shows section-level leaves (Devices, Color Strips, LED Targets,
KC Targets, Pattern Templates) with scroll-to-section on click.
Sources tree groups into Picture, Audio, and Utility categories.

Also fixes missing csAudioTemplates in stream section expand/collapse.

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

server/src/wled_controller/static/css/streams.css

@@ -700,6 +700,7 @@ body.pp-filter-dragging .pp-filter-drag-handle {
 /* Sub-tab content sections */
 .subtab-section {
     margin-bottom: 24px;
+    scroll-margin-top: var(--header-height, 48px);
 }
 
 .subtab-section:last-child {