Support non-0.4mm nozzles in AMS Slot config + guard dispatch (#1899)

The Configure AMS Slot picker was hardwired to 0.4mm (nozzleDiameter
prop never passed from PrintersPage / SpoolBuddyAmsPage), so a 0.6
machine could only set 0.4 profiles on its trays. Resolve the real
installed nozzle per-AMS (ams_extruder_map on dual-nozzle) and pass it
in. Separately, nothing validated the sliced nozzle against the
installed one, so a mismatch reached the printer as a cryptic HMS
_8012 "Failed to get AMS mapping table". Add a fail-safe pre-dispatch
guard in _start_print that fails the item with an actionable message
before upload; no slice diameter or no reported nozzles = no-op.

frontend/src/__tests__/utils/resolveSlotNozzleDiameter.test.ts

@@ -0,0 +1,63 @@
+/**
+ * Tests for resolveSlotNozzleDiameter helper (#1899).
+ *
+ * The AMS Slot config picker must filter filament presets by the nozzle that
+ * actually feeds a given AMS, not the hardcoded 0.4mm default. This resolver
+ * reads the installed nozzle diameter from the printer status, honouring the
+ * per-AMS extruder binding on dual-nozzle printers. It returns undefined when
+ * the hardware hasn't been reported, so the caller keeps its own default.
+ */
+
+import { describe, it, expect } from 'vitest';
+
+import { resolveSlotNozzleDiameter } from '../../utils/amsHelpers';
+
+describe('resolveSlotNozzleDiameter', () => {
+  it('returns undefined when status is null or undefined', () => {
+    expect(resolveSlotNozzleDiameter(null, 0)).toBeUndefined();
+    expect(resolveSlotNozzleDiameter(undefined, 0)).toBeUndefined();
+  });
+
+  it('returns undefined when no nozzles are reported', () => {
+    expect(resolveSlotNozzleDiameter({ nozzles: [] }, 0)).toBeUndefined();
+    expect(resolveSlotNozzleDiameter({}, 0)).toBeUndefined();
+  });
+
+  it('returns undefined when the reported nozzle diameter is an empty default', () => {
+    expect(resolveSlotNozzleDiameter({ nozzles: [{ nozzle_diameter: '' }] }, 0)).toBeUndefined();
+  });
+
+  it('returns the single-nozzle diameter regardless of amsId (no extruder map)', () => {
+    const status = { nozzles: [{ nozzle_diameter: '0.6' }] };
+    expect(resolveSlotNozzleDiameter(status, 0)).toBe('0.6');
+    expect(resolveSlotNozzleDiameter(status, 3)).toBe('0.6');
+  });
+
+  it('resolves the per-AMS nozzle on a dual-nozzle printer via ams_extruder_map', () => {
+    // AMS 0 → left nozzle (0.4), AMS 1 → right nozzle (0.6)
+    const status = {
+      nozzles: [{ nozzle_diameter: '0.4' }, { nozzle_diameter: '0.6' }],
+      ams_extruder_map: { '0': 0, '1': 1 },
+    };
+    expect(resolveSlotNozzleDiameter(status, 0)).toBe('0.4');
+    expect(resolveSlotNozzleDiameter(status, 1)).toBe('0.6');
+  });
+
+  it('falls back to the primary nozzle when the AMS is not in the extruder map', () => {
+    const status = {
+      nozzles: [{ nozzle_diameter: '0.4' }, { nozzle_diameter: '0.6' }],
+      ams_extruder_map: { '0': 0 },
+    };
+    // AMS 5 has no mapping → index 0 (primary)
+    expect(resolveSlotNozzleDiameter(status, 5)).toBe('0.4');
+  });
+
+  it('falls back to the primary nozzle when the mapped nozzle has no diameter', () => {
+    // Dual-nozzle stub where the second entry is still an empty default.
+    const status = {
+      nozzles: [{ nozzle_diameter: '0.4' }, { nozzle_diameter: '' }],
+      ams_extruder_map: { '1': 1 },
+    };
+    expect(resolveSlotNozzleDiameter(status, 1)).toBe('0.4');
+  });
+});