UI: logo book fill light mode black → indigo

.logo-book fill was var(--color-text) which renders black in light mode.
Changed to var(--color-primary) so it matches the brand indigo in both
modes. Stroke updated to match at 70% opacity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AppLogo.vue

@@ -37,8 +37,8 @@ defineProps<{ size?: number }>();
 
 <style scoped>
 .logo-book {
-  fill: var(--color-text);
-  stroke: var(--color-text-secondary);
+  fill: var(--color-primary);
+  stroke: color-mix(in srgb, var(--color-primary) 70%, transparent);
 }
 .logo-spine {
   stroke: var(--color-text-secondary);