feat: Add color column with filter to spool list

Add optional color column to spool list showing filament color swatches
with multi-select filtering capability.

Backend:
- GET /api/v1/color endpoint to list distinct filament colors
- filament.color_hex query param on spool list for filtering

Frontend:
- ColorFilterColumn component renders color swatches
- useSpoolmanColors() hook fetches available colors
- Filter dropdown shows swatches with hex codes
- Column hidden by default (enable via column selector)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/public/locales/en/common.json

@@ -173,6 +173,7 @@
             "filament_internal": "Internal",
             "filament_external": "External",
             "price": "Price",
+            "color": "Color",
             "material": "Material",
             "weight_to_use": "Weight",
             "used_weight": "Used Weight",