fix: marker template was overriding the red default with --accent

The inline style wrote --marker-color: var(--accent) whenever
marker.color wasn't set, which shadowed the CSS default. Only emit
the inline override when a per-marker color is explicitly set so the
stylesheet's red (#ef4444) actually applies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Timeline.vue

@@ -2001,7 +2001,7 @@ onBeforeUnmount(() => {
               :class="{ selected: selectedMarkerId === marker.id }"
               :style="{
                 left: pxFromTime(marker.time) + 'px',
-                '--marker-color': marker.color || 'var(--accent)',
+                ...(marker.color ? { '--marker-color': marker.color } : {}),
               }"
               :title="marker.name"
               @pointerdown="onMarkerPointerDown($event, marker.id)"