Update POST JSON size limit to use SAVEGAME_SLOT_SIZE minus header space

Software/MainBoard/rust/src/webserver/post_json.rs

@@ -1,5 +1,6 @@
 use crate::config::PlantControllerConfig;
 use crate::fat_error::FatResult;
+use crate::hal::savegame_manager::SAVEGAME_SLOT_SIZE;
 use crate::hal::DetectionRequest;
 use crate::webserver::read_up_to_bytes_from_request;
 use crate::{do_secure_pump, BOARD_ACCESS};
@@ -135,7 +136,8 @@ pub(crate) async fn set_config<T, const N: usize>(
 where
     T: Read + Write,
 {
-    let all = read_up_to_bytes_from_request(request, Some(4096)).await?;
+    //accept nearly full slotsize leave some space for header
+    let all = read_up_to_bytes_from_request(request, Some(SAVEGAME_SLOT_SIZE - 512)).await?;
     let length = all.len();
     let config: PlantControllerConfig = serde_json::from_slice(&all)?;