refactor(color-strip): rename static -> single + frontend follow-through

The "static" source kind always rendered a SINGLE color and the name
confused new code paths. Rename the module + kind to "single". Storage
keeps backward-compatible serialisation. Frontend color-strip cards /
gradient / index / test modules and the affected tests follow the new
name.

server/tests/test_composite_nesting.py

@@ -22,8 +22,8 @@ def store(tmp_db):
 
 
 def _create_static(store: ColorStripStore, name: str) -> str:
-    """Create a static color strip source, return its ID."""
-    source = store.create_source(name=name, source_type="static", colors=[[255, 0, 0]])
+    """Create a single-color strip source, return its ID."""
+    source = store.create_source(name=name, source_type="single_color", colors=[[255, 0, 0]])
     return source.id