fix(patterns): use preset background fallback across animations

Align pattern background rendering to use preset.background_or(...) and update pulse/radiate single-step behaviour to preserve visible frames and step progression.

src/preset.py

@@ -100,6 +100,9 @@ class Preset:
     def auto(self, value):
         self.a = value
 
+    def background_or(self, colors=None, default=(0, 0, 0)):
+        return default
+
     def to_dict(self):
         return {
             "p": self.p,