fix(targets): accept a child's weight target below 30 kg

A weight target was refused below 30 kg on every save path (the own
targets editor and a managed record's settings parse the same schema),
while a weight reading is accepted from 1 kg. A guardian setting 20 to
23 kg for a child got only "please check your entries". The target
floor is now the measurement floor.

The weight bands on the dashboard and the weight page started their low
red zone at a hard-coded 30 kg, so a target below it lost that zone; the
edge now follows the same floor. Bands are clipped to the chart's own
domain, so an adult's chart looks the same. `buildWeightBandsFromHeight`
had no caller left and goes; the band builder's 30/250 defaults, which
every caller overrode, go with it. The unit-adapter test keeps its
inward-rounding check at the new floor (1 kg is 2.2046 lb, offered as
2.3).

src/__tests__/display-transform-rounding-guard.test.ts

@@ -45,8 +45,9 @@ const REGISTRY = "lib/measurements/display-transform.ts";
 /**
  * The one module allowed to import the unrounded escape hatch. The target
  * adapter needs the unrounded pound edge of a canonical guardrail so it can
- * round it INWARD — rounding to the nearest tenth first gives 66.1 lb for the
- * 30 kg floor, which inverts to 29.98 kg and the server rejects it.
+ * round it INWARD — rounding to the nearest tenth first can put the pound
+ * edge outside the kilogram window, and the server rejects what converts
+ * back from it.
  */
 const UNROUNDED_IMPORTERS = ["lib/targets/target-unit-display.ts"];