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>

spoolman/database/filament.py

@@ -241,6 +241,16 @@ async def find_materials(
     return [row[0] for row in rows.all() if row[0] is not None]
 
 
+async def find_colors(
+    *,
+    db: AsyncSession,
+) -> list[str]:
+    """Find a list of unique filament colors (hex codes) from the filament table."""
+    stmt = select(models.Filament.color_hex).distinct()
+    rows = await db.execute(stmt)
+    return sorted([row[0] for row in rows.all() if row[0] is not None])
+
+
 async def find_article_numbers(
     *,
     db: AsyncSession,