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>

prisma/migrations/20260707012217_v0_28_root_vegetable_category/migration.sql

@@ -0,0 +1,2 @@
+-- AlterEnum
+ALTER TYPE "PlantCategory" ADD VALUE 'ROOT_VEGETABLE';