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/schema.prisma

@@ -122,6 +122,7 @@ enum PlantCategory {
   GROUNDCOVER    // strawberries, clover, creeping thyme
   VINE           // grapes, hops, kiwi
   ANNUAL_VEGETABLE  // tomatoes, squash, beans, peppers — seasonal veg
+  ROOT_VEGETABLE    // carrots, beets, sweet potatoes, radishes
   ANNUAL            // legacy, treated as annual vegetable
   ANNUAL_FLOWER     // zinnias, marigolds, nasturtiums, cosmos
   PERENNIAL         // asparagus, rhubarb, perennial veg