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.

static/index.html

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