feat(slice-modal): Bundle tier for picking presets from imported .bbscfg

Closes the loop on the bundle work: users who imported a Printer
  Preset Bundle via Settings → Slicer Bundles can now pick it in the
  SliceModal and slice through the bundle dispatch path the backend
  already supports.

  UX:
  - New "Slicer bundle" picker at the top of the modal, rendered only
    when at least one bundle is imported (GET /slicer/bundles non-empty)
  - Selecting a bundle replaces cloud/local/standard preset dropdowns
    with bundle-scoped pickers (process + per-slot filament names from
    the bundle). Printer is implicit (each .bbscfg has exactly one).
  - Submit routes through SliceRequest.bundle so the backend skips
    PresetRef resolution and asks the sidecar to materialise the JSON
    triplet from the stored bundle by name.
  - "None" leaves the modal on the original preset triplet path.

  Frontend types: SliceBundleSpec + bundle?: SliceBundleSpec on SliceRequest.

frontend/src/api/client.ts

@@ -1155,6 +1155,13 @@ export interface PresetRef {
   source: PresetSource;
   id: string;
 }
+export interface SliceBundleSpec {
+  bundle_id: string;
+  printer_name: string;
+  process_name: string;
+  // Per-slot filament names in plate order. Index 0 = slot 1, etc.
+  filament_names: string[];
+}
 export interface SliceRequest {
   printer_preset_id?: number;
   process_preset_id?: number;
@@ -1167,6 +1174,11 @@ export interface SliceRequest {
   // backend validator promotes a singular into a one-element list when this
   // is omitted, so legacy single-color clients keep working unchanged.
   filament_presets?: PresetRef[];
+  // Bundle dispatch: when set, the backend skips PresetRef resolution and
+  // picks the JSON triplet from a sidecar-stored .bbscfg by name. Mutually
+  // exclusive with the preset fields above (validator accepts both, but
+  // dispatch ignores the preset side when bundle is set).
+  bundle?: SliceBundleSpec;
   plate?: number;
   export_3mf?: boolean;
 }