Address review: redact plant owner ids, cache-control, 400, DRY reset

- Security (2-model, security+correctness): the public payload zeroed
  Garden.OwnerID but referenced custom plants still carried Plant.OwnerID,
  leaking the owner's user id to anonymous viewers. Redact plant owner ids
  too, and assert it in the service test.
- Set Cache-Control: no-store on the public read so a capability-URL response
  isn't held in a shared proxy cache and always reflects the live garden.
- createShareLink now 400s on a present-but-malformed JSON body instead of
  silently enabling (an empty body still means enable-without-rotate).
- Extract the transient-editor-state reset into a shared resetTransient store
  action (3-model finding) and use it from PublicGardenPage.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/service/public_test.go

@@ -84,6 +84,13 @@ func TestPublicShareLink(t *testing.T) {
 		t.Errorf("public full = %d objs / %d plops / %d plants, want 1/1/1",
 			len(full.Objects), len(full.Plantings), len(full.Plants))
 	}
+	// The referenced plant is the owner's custom plant; its OwnerID must be
+	// redacted so an anonymous viewer can't learn the owner's user id.
+	for _, pl := range full.Plants {
+		if pl.OwnerID != nil {
+			t.Errorf("public plant %d leaks OwnerID %d, want nil", pl.ID, *pl.OwnerID)
+		}
+	}
 
 	// Unknown / blank tokens are masked, never a distinct error.
 	for _, bad := range []string{"does-not-exist", "  ", ""} {