Fix filter_template expansion in test routes and select defaults

filter_template references were silently ignored in PP template test,
picture source test, and KC target test routes — they created a no-op
FilterTemplateFilter instead of expanding into the referenced template's
filters. Centralized expansion logic into PostprocessingTemplateStore.
resolve_filter_instances() and use it in all test routes + live stream
manager.

Also fixed empty template_id when adding filter_template filters: the
select dropdown showed the first template visually but onchange never
fired, saving "" instead. Now initializes with first choice's value and
auto-corrects stale/empty values at render time.

Other fixes: ScreenCapture dimensions now use actual image shape after
filter processing; brightness source label emoji updates.

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

server/src/wled_controller/api/routes/picture_targets.py

@@ -578,7 +578,8 @@ async def test_kc_target(
                 except ValueError:
                     logger.warning(f"KC test: PP template {pp_id} not found, skipping")
                     continue
-                for fi in pp_template.filters:
+                flat_filters = pp_template_store.resolve_filter_instances(pp_template.filters)
+                for fi in flat_filters:
                     try:
                         f = FilterRegistry.create_instance(fi.filter_id, fi.options)
                         result = f.process_image(img_array, image_pool)