web/board: lift card label chips off the dark card surface

The chip's stock surface-container-high equals the dark theme's card
surface, so label pills blended into the card. Mix the background from
on-surface instead, keeping the pills one visible step off the card in
both themes.

Claude-Session: https://claude.ai/code/session_01WBGqifASAjytHQNPGdcQQg

Author: yorukot

web/src/features/board/CardLabels.module.css

@@ -16,4 +16,8 @@
 .chip {
 	flex: 0 0 auto;
 	white-space: nowrap;
+	/* The chip's stock surface-container-high matches the dark card surface
+	 * exactly, which made the label pills vanish there. Mixing from on-surface
+	 * keeps them one visible step off the card in both themes. */
+	background: color-mix(in srgb, var(--md-sys-color-on-surface) 8%, transparent);
 }