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_sources.py

@@ -417,19 +417,21 @@ async def test_picture_source(
         if pp_template_ids:
             try:
                 pp_template = pp_store.get_template(pp_template_ids[0])
-                pool = ImagePool()
+                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:
-                        f = FilterRegistry.create_instance(fi.filter_id, fi.options)
-                        result = f.process_image(arr, pool)
-                        if result is not None:
-                            arr = result
-                    return Image.fromarray(arr)
+                    def apply_filters(img):
+                        arr = np.array(img)
+                        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:
+                                arr = result
+                        return Image.fromarray(arr)
 
-                thumbnail = apply_filters(thumbnail)
-                pil_image = apply_filters(pil_image)
+                    thumbnail = apply_filters(thumbnail)
+                    pil_image = apply_filters(pil_image)
             except ValueError:
                 logger.warning(f"PP template {pp_template_ids[0]} not found, skipping postprocessing preview")