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/storage/postprocessing_template_store.py

@@ -220,6 +220,30 @@ class PostprocessingTemplateStore:
 
         logger.info(f"Deleted postprocessing template: {template_id}")
 
+    def resolve_filter_instances(self, filter_instances, _visited=None):
+        """Recursively resolve filter instances, expanding filter_template references.
+
+        Returns a flat list of FilterInstance objects with no filter_template entries.
+        """
+        if _visited is None:
+            _visited = set()
+        resolved = []
+        for fi in filter_instances:
+            if fi.filter_id == "filter_template":
+                template_id = fi.options.get("template_id", "")
+                if not template_id or template_id in _visited:
+                    continue
+                try:
+                    ref_template = self.get_template(template_id)
+                    _visited.add(template_id)
+                    resolved.extend(self.resolve_filter_instances(ref_template.filters, _visited))
+                    _visited.discard(template_id)
+                except ValueError:
+                    logger.warning(f"Referenced filter template '{template_id}' not found, skipping")
+            else:
+                resolved.append(fi)
+        return resolved
+
     def get_sources_referencing(self, template_id: str, picture_source_store) -> List[str]:
         """Return names of picture sources that reference this template."""
         return [