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/store/users.go

@@ -19,12 +19,6 @@ type scanner interface {
 	Scan(dest ...any) error
 }
 
-// queryer is the read surface shared by *sql.DB and *sql.Tx, so a list helper can
-// serve both a standalone read and one inside a transaction.
-type queryer interface {
-	QueryContext(ctx context.Context, query string, args ...any) (*sql.Rows, error)
-}
-
 // scanUser reads one users row. is_admin is stored as INTEGER 0/1 (the driver
 // returns it as int64, which does not convert straight to bool), so it is read
 // into an int64 and mapped.