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>

lib/format.js

@@ -21,8 +21,15 @@ export function formatBytes(n) {
 }
 
 // Compact for the panel: "2.4M", "120K", "0".
+// Keeps the mantissa under three integer digits so a value is at most four
+// characters wide ("999K", "9.9M", "1023") — the panel field has a fixed width
+// and must never overflow it (see .sbx-speed-label in stylesheet.css).
 export function formatBytesShort(n) {
-    const [v, i] = scale(n);
+    let [v, i] = scale(n);
+    if (i > 0 && v >= 1000 && i < SHORT_UNITS.length - 1) {
+        v /= 1024;
+        i += 1;
+    }
     if (i === 0)
         return `${Math.round(v)}`;
     const digits = v < 10 ? 1 : 0;