Show Spoolman fill level for AMS Lite and external spools (#293)

AMS Lite units (A1 series) have no weight sensor and always report 0%
fill level. When a spool is linked to Spoolman with weight data, use
Spoolman's remaining weight as a fallback. External spools also show
fill level from Spoolman data instead of always showing unknown.

Backend: Enrich GET /spoolman/spools/linked response with
remaining_weight and filament_weight alongside spool ID.

Frontend: Add getSpoolmanFillLevel() helper. Update regular AMS, HT
AMS, and external spool fill computations to use Spoolman fallback
when AMS reports 0%. Show "(Spoolman)" indicator in hover card when
fill data comes from Spoolman.

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

@@ -0,0 +1,78 @@
+/**
+ * Tests for getSpoolmanFillLevel helper function.
+ * This function is defined in PrintersPage.tsx but tested here for isolation.
+ * We replicate the logic to test it independently.
+ */
+
+import { describe, it, expect } from 'vitest';
+
+// Replicate the function from PrintersPage.tsx for testing
+interface LinkedSpoolInfo {
+  id: number;
+  remaining_weight: number | null;
+  filament_weight: number | null;
+}
+
+function getSpoolmanFillLevel(
+  linkedSpool: LinkedSpoolInfo | undefined
+): number | null {
+  if (!linkedSpool?.remaining_weight || !linkedSpool?.filament_weight
+      || linkedSpool.filament_weight <= 0) return null;
+  return Math.min(100, Math.round(
+    (linkedSpool.remaining_weight / linkedSpool.filament_weight) * 100
+  ));
+}
+
+describe('getSpoolmanFillLevel', () => {
+  it('returns null for undefined spool', () => {
+    expect(getSpoolmanFillLevel(undefined)).toBeNull();
+  });
+
+  it('returns null when remaining_weight is null', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: null, filament_weight: 1000 })).toBeNull();
+  });
+
+  it('returns null when filament_weight is null', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 500, filament_weight: null })).toBeNull();
+  });
+
+  it('returns null when remaining_weight is 0', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 0, filament_weight: 1000 })).toBeNull();
+  });
+
+  it('returns null when filament_weight is 0', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 500, filament_weight: 0 })).toBeNull();
+  });
+
+  it('returns null when filament_weight is negative', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 500, filament_weight: -100 })).toBeNull();
+  });
+
+  it('calculates correct percentage for half-full spool', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 500, filament_weight: 1000 })).toBe(50);
+  });
+
+  it('calculates correct percentage for full spool', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 1000, filament_weight: 1000 })).toBe(100);
+  });
+
+  it('calculates correct percentage for nearly empty spool', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 50, filament_weight: 1000 })).toBe(5);
+  });
+
+  it('caps at 100% when remaining exceeds filament weight', () => {
+    // This can happen if user manually sets remaining_weight higher
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 1200, filament_weight: 1000 })).toBe(100);
+  });
+
+  it('rounds to nearest integer', () => {
+    // 333/1000 = 33.3% -> 33%
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 333, filament_weight: 1000 })).toBe(33);
+    // 666/1000 = 66.6% -> 67%
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 666, filament_weight: 1000 })).toBe(67);
+  });
+
+  it('handles small weights correctly', () => {
+    expect(getSpoolmanFillLevel({ id: 1, remaining_weight: 1, filament_weight: 100 })).toBe(1);
+  });
+});