Fix component-source lookup: prefix ~ for component env key

Components are stored as ~name in the env. The helper was looking up
bare name without the tilde prefix.

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

sx/sxc/pages/helpers.py

@@ -46,7 +46,8 @@ def _component_source(name: str) -> str:
     from shared.sx.types import Component, Island
     from shared.sx.ref.sx_ref import build_component_source
 
-    comp = get_component_env().get(name)
+    key = name if name.startswith("~") else f"~{name}"
+    comp = get_component_env().get(key)
     if isinstance(comp, Island):
         return build_component_source({
             "type": "island", "name": name,