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/patterns/strobe_burst.py

@@ -16,7 +16,7 @@ class StrobeBurst:
             cooldown = max(1, int(preset.n3) if int(preset.n3) > 0 else 400)
             on_ms = max(1, int(preset.d) // 2)
             c = self.driver.apply_brightness(colors[0], preset.b)
-            bg_color = self.driver.apply_brightness(colors[-1], preset.b)
+            bg_color = self.driver.apply_brightness(preset.background_or(colors), preset.b)
             now = utime.ticks_ms()
 
             if state == "flash_on":