v0.28.0 — Fix null toxicToPets on type create; Root vegetable category; sweet potato knowledge

The plant-type CREATE schema rejected toxicToPets: null (the dialog's
'Unknown'), failing every manually-entered type with 'expected boolean,
received null' — now nullable like the PATCH schema always was. New
ROOT_VEGETABLE PlantCategory (enum migration + labels/order). Garden
brain gains Sweet potato (slips in June, dig before frost, warm cure),
and knowledgeFor/tipsForMonth now prefer the most specific name match so
'Japanese sweet potato' gets sweet-potato advice instead of Potato's
nightshade rotation and hilling (regression-tested).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/garden/categories.ts

@@ -9,6 +9,7 @@ export const CATEGORY_LABELS: Record<PlantCategory, string> = {
   GROUNDCOVER:      "Groundcover",
   VINE:             "Vine",
   ANNUAL_VEGETABLE: "Annual vegetable",
+  ROOT_VEGETABLE:   "Root vegetable",
   ANNUAL:           "Annual vegetable",
   ANNUAL_FLOWER:    "Annual flower",
   PERENNIAL:        "Perennial",
@@ -29,6 +30,7 @@ export const CATEGORY_ORDER: PlantCategory[] = [
   "PERENNIAL",
   "PERENNIAL_FLOWER",
   "ANNUAL_VEGETABLE",
+  "ROOT_VEGETABLE",
   "ANNUAL_FLOWER",
   "BULB",
   "MUSHROOM",