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/clock_sweep.py

@@ -15,7 +15,7 @@ class ClockSweep:
             d = max(1, int(preset.d))
             now = utime.ticks_ms()
             if utime.ticks_diff(now, last) >= d:
-                bg = self.driver.apply_brightness(colors[-1], preset.b)
+                bg = self.driver.apply_brightness(preset.background_or(colors), preset.b)
                 fg = self.driver.apply_brightness(colors[0], preset.b)
                 for i in range(self.driver.num_leds):
                     self.driver.n[i] = bg