Stabilize panel speed label width

Cap formatBytesShort at four characters by rolling the mantissa up to the
next unit once it reaches 1000, and give .sbx-speed-label a fixed min-width
so the panel widget no longer shifts horizontally as speed values change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

stylesheet.css

@@ -15,6 +15,10 @@
 .sbx-speed-label {
     font-feature-settings: "tnum";
     font-size: 0.9em;
+    /* Fixed width so the panel widget doesn't shift as speed values change.
+       Sized for the widest field: "↓1023 ↑1023" (see formatBytesShort). */
+    min-width: 7.5em;
+    text-align: left;
 }
 
 .sbx-status-item {