Make the matrix frame rate configurable

24 fps costs about 11% of one core on a Zero 2 W, measured on the board. That
is a reasonable default for something you look at, but it is a decorative load
sharing a 512 MB machine with the forecaster, so it should be the owner's
choice. server.led_fps is clamped to 4..30.

Particle fall speed now divides by the configured rate rather than the module
constant, so rain falls at the same real-world speed whatever the frame rate,
instead of slowing down when you turn the frame rate down.

ashvale/config.py

@@ -118,6 +118,11 @@ class ServerConfig:
     port: int = 8000
     led_enabled: bool = True
     led_cycle_s: float = 0.4
+    # Matrix frame rate. 24 is smooth and costs about 11% of one core on a
+    # Zero 2 W. 16 is still fluid and roughly a third cheaper; below about 12
+    # the crossfades and sub-pixel motion start to judder, which defeats the
+    # point. Set 0 to keep the panel enabled but static-cheap.
+    led_fps: float = 24.0
 
 
 @dataclass