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/postprocessing.py

@@ -283,13 +283,14 @@ async def test_pp_template(
         thumbnail = pil_image.copy()
         thumbnail.thumbnail((thumbnail_width, thumbnail_height), Image.Resampling.LANCZOS)
 
-        # Apply postprocessing filters
-        if pp_template.filters:
+        # Apply postprocessing filters (expand filter_template references)
+        flat_filters = pp_store.resolve_filter_instances(pp_template.filters)
+        if flat_filters:
             pool = ImagePool()
 
             def apply_filters(img):
                 arr = np.array(img)
-                for fi in pp_template.filters:
+                for fi in flat_filters:
                     f = FilterRegistry.create_instance(fi.filter_id, fi.options)
                     result = f.process_image(arr, pool)
                     if result is not None: