feat(slicing): build-plate override in the SliceModal (#1337)

Slicing an STL via the integrated slicer always defaulted to whatever
  curr_bed_type lived in the chosen process preset (typically "Cool
  Plate"), which the slicer CLI rejected for high-temp filaments with
  "Plate 1: Cool Plate does not support filament 1". The user had no
  way to switch plates without cloning the preset in BambuStudio.

  The Slice modal now exposes a Build plate dropdown with the six
  canonical BambuStudio / OrcaSlicer plates (Cool Plate, Cool Plate
  SuperTack, Engineering Plate, High Temp Plate, Textured PEI Plate,
  Smooth PEI Plate) plus an "Auto (use process preset)" option that
  preserves the previous behavior. Positioned between Process profile
  and Filament rows so a long filament list never pushes it off the
  modal's scrolled viewport, and always enabled regardless of whether
  the user picked a Printer Preset Bundle.

  A new bed_type field on SliceRequest flows through both dispatch
  paths:
  - Resolved-preset path: _patch_process_bed_type overwrites
    curr_bed_type on the process JSON before forwarding to the sidecar.
    Works end-to-end today, no sidecar change needed.
  - Bundle dispatch path: slice_with_bundle adds a bedType form field
    to the sidecar multipart. The sidecar (maziggy/orca-slicer-api
    fork) needs a matching change to honor it as --curr_bed_type on
    the CLI invocation; until then the field is silently ignored and
    the slice runs with the bundle's default plate.

backend/app/services/slicer_api.py

@@ -435,6 +435,7 @@ class SlicerApiService:
         filament_names: list[str],
         plate: int | None = None,
         export_3mf: bool = False,
+        bed_type: str | None = None,
         request_id: str | None = None,
         on_progress: Callable[[dict], None] | None = None,
     ) -> SliceResult:
@@ -476,6 +477,16 @@ class SlicerApiService:
             data["plate"] = str(plate)
         if export_3mf:
             data["exportType"] = "3mf"
+        if bed_type is not None:
+            # #1337: bed-plate override flows through to the sidecar as a
+            # standalone field. The sidecar wraps this as --curr_bed_type on
+            # the CLI invocation, overriding whatever the bundle's process
+            # JSON specifies. Bambuddy can't patch the bundle's JSON locally
+            # (the sidecar materialises it from disk), so this round-trip is
+            # the only path. Silently no-ops on sidecar versions that don't
+            # yet recognise the field — the user's slice still runs with the
+            # bundle's default plate, no crash.
+            data["bedType"] = bed_type
         if request_id is not None:
             data["requestId"] = request_id