Run cargo fmt on the unit code and drop a quantity box's invalid mark when its line is removed

The new CI runs cargo fmt --check. QuantityInput also reported its validity only when it changed, so removing a line
whose quantity box held bad text left that box counted as invalid and blocked issuing; it now reports valid when it
unmounts, and no longer writes a ref during render.

Claude-Session: https://claude.ai/code/session_01PZypiWDfMkDTeEPeXjRhW5

src/components/QuantityInput.tsx

@@ -39,8 +39,15 @@ export default function QuantityInput({ id, value, unit, onChange, disabled, onV
   const parsed = parseQuantity(text, unit);
   const clock = knownUnit(unit)?.clock;
   const validity = useRef(onValidity);
-  validity.current = onValidity;
-  useEffect(() => validity.current?.(parsed.ok), [parsed.ok]);
+  useEffect(() => {
+    validity.current = onValidity;
+  });
+  // Reported while this box exists; a box that goes away (its line was removed) stops counting as invalid.
+  useEffect(() => {
+    const report = validity.current;
+    report?.(parsed.ok);
+    return () => report?.(true);
+  }, [parsed.ok]);
 
   const helper = parsed.ok && text.includes(":") ? `= ${formatQuantity(parsed.value, "unit")} ${knownUnit(unit)?.short ?? ""}`.trim() : undefined;