Address Gadfly review on the garden copy

* api: detect an absent copy body by io.EOF from the decoder rather than
  Content-Length. A chunked request reports -1, not 0, so the length test
  read an empty chunked body as malformed and 400'd instead of taking the
  default-name path. Covered by a new unknown-length test.

* store: hoist the garden_objects/plantings INSERTs into shared
  objectInsert/plantingInsert statements with matching *InsertArgs
  helpers. CopyGarden had its own copies of both column lists, so a
  column added to the table could be wired into Create* and silently
  dropped from a copy.

* store: move the queryer interface to sqlite.go, next to the other
  shared query plumbing, instead of users.go.

* web: derive a copy's prefilled name via defaultCopyName, mirroring the
  server's copyName including its 200-BYTE cap. The inline
  `${name} (copy)` both duplicated the suffix and, for a garden whose
  name was already at the cap, prefilled an over-long name that the
  server rejected with a 400. Unit-tested, including multi-byte
  truncation on a code-point boundary.

* test: drop a throwaway `_ = kept`.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

internal/api/api.go

@@ -81,7 +81,7 @@ func New(cfg *config.Config, svc *service.Service) *gin.Engine {
 	gardens.GET("/:id", h.getGarden)
 	gardens.PATCH("/:id", h.updateGarden)
 	gardens.DELETE("/:id", h.deleteGarden)
-	gardens.POST("/:id/copy", h.copyGarden) // duplicate a garden the actor owns
+	gardens.POST("/:id/copy", h.copyGarden)   // duplicate a garden the actor owns
 	gardens.GET("/:id/full", h.getGardenFull) // one-shot editor load
 	gardens.POST("/:id/objects", h.createObject)