fix(labs): keep a reading's conversion record unless its value actually changes

The single-reading edit cleared sourceValue and sourceUnit whenever the body
carried a value, and the web edit sheet always sends one, so a date or note
edit erased the "converted from" hint. It is now cleared only when the value
differs from the stored one. The sheet also opened a converted reading at its
stored precision (100.90191999999999); it now starts from the value as the lab
surfaces show it and leaves an untouched value out of the request, so the
stored full-precision number is not replaced by its rounded display.

src/app/api/labs/[id]/route.ts

@@ -195,8 +195,14 @@ export const PUT = apiHandler(
     if (d.value !== undefined) {
       data.value = d.value;
       // A number the person typed is no longer the conversion of what the
-      // report printed, so the "converted from" record would now be wrong.
-      if (existing.sourceValue !== null || existing.sourceUnit !== null) {
+      // report printed, so the "converted from" record would now be wrong. Only
+      // a CHANGED value says so: a client that sends the whole form back with
+      // the value untouched (the web edit dialog does) must not erase the
+      // record. Both sides are plain numbers (`value` is a Float column).
+      if (
+        d.value !== existing.value &&
+        (existing.sourceValue !== null || existing.sourceUnit !== null)
+      ) {
         data.sourceValue = null;
         data.sourceUnit = null;
       }