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/locales/en.json

@@ -1083,6 +1083,8 @@
   "audio_template.error.delete": "Failed to delete audio template",
   "streams.group.value": "Value Sources",
   "streams.group.sync": "Sync Clocks",
+  "tree.group.picture": "Picture",
+  "tree.group.utility": "Utility",
   "value_source.group.title": "Value Sources",
   "value_source.add": "Add Value Source",
   "value_source.edit": "Edit Value Source",