client_v2: fix crash inspecting a spool of a colorless filament

A filament with neither color_hex nor multi_color_hexes maps to an empty
colors array, and the spool inspector's color-hex row indexed [0]
unconditionally, throwing on render. Show the usual "—" placeholder
instead.

Claude-Session: https://claude.ai/code/session_014Cwk5Yyze9yPvkYkUtMAP4

client_v2/src/lib/components/library/SpoolInspector.svelte

@@ -332,7 +332,7 @@
 						<span class="mono"
 							>{filament.colors.length > 1
 								? m['inspector.multi']({ count: filament.colors.length })
-								: filament.colors[0].toUpperCase()}</span
+								: (filament.colors[0]?.toUpperCase() ?? '—')}</span
 						>
 					</span>
 				</Field>