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/components/FilamentHoverCard.tsx

@@ -10,6 +10,7 @@ interface FilamentData {
   kFactor: string;
   fillLevel: number | null; // null = unknown
   trayUuid?: string | null; // Bambu Lab spool UUID for Spoolman linking
+  fillSource?: 'ams' | 'spoolman'; // Source of fill level data
 }
 
 interface SpoolmanConfig {
@@ -229,8 +230,11 @@ export function FilamentHoverCard({ data, children, disabled, className = '', sp
                     <Droplets className="w-3 h-3" />
                     {t('ams.fill')}
                   </span>
-                  <span className="text-xs text-white font-semibold">
+                  <span className="text-xs text-white font-semibold flex items-center gap-1">
                     {data.fillLevel !== null ? `${data.fillLevel}%` : '—'}
+                    {data.fillSource === 'spoolman' && data.fillLevel !== null && (
+                      <span className="text-[9px] text-bambu-gray font-normal">{t('spoolman.fillSourceLabel')}</span>
+                    )}
                   </span>
                 </div>
                 {/* Fill bar */}