fix(patterns): correct non-blocking timing and blink off phase

Co-authored-by: Cursor <cursoragent@cursor.com>

src/patterns/snowfall.py

@@ -16,10 +16,11 @@ class Snowfall:
             d = max(1, int(preset.d))
             now = utime.ticks_ms()
             if utime.ticks_diff(now, last) >= d:
+                bg_color = self.driver.apply_brightness(colors[-1], preset.b)
                 if random.randint(0, 255) < density:
                     flakes.append([self.driver.num_leds - 1, random.randint(0, len(colors)-1)])
                 for i in range(self.driver.num_leds):
-                    self.driver.n[i] = (0, 0, 0)
+                    self.driver.n[i] = bg_color
                 nf = []
                 for pos, ci in flakes:
                     if 0 <= pos < self.driver.num_leds: