style(flex): a little more room in the chaser's offset box

Narrowed to w-12 earlier, which is tight for a signed four-digit offset:
-1500 filled it edge to edge with no padding left.

frontend/src/components/FlexPanel.tsx

@@ -725,7 +725,7 @@ export function FlexPanel({ onCWSpeed, onReportRST }: { onCWSpeed?: (wpm: number
                       const next = { ...rstChase, offset_hz: v };
                       setRstChase(next); SaveFlexRSTChase(next as any).catch(() => {});
                     }}
-                    className="w-12 h-6 rounded border border-input bg-background px-1 text-[11px] font-mono" />
+                    className="w-16 h-6 rounded border border-input bg-background px-1.5 text-[11px] font-mono" />
                   Hz
                 </label>
               )}