feat(theme): shift palette from indigo to deep violet + muted gold

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

frontend/src/components/AppLogo.vue

@@ -11,6 +11,12 @@ defineProps<{ size?: number }>();
     :height="size ?? 24"
     aria-hidden="true"
   >
+    <defs>
+      <linearGradient id="logo-gradient" x1="0" y1="0" x2="1" y2="1">
+        <stop offset="0%" stop-color="var(--color-primary-solid)" />
+        <stop offset="100%" stop-color="var(--color-primary-deep)" />
+      </linearGradient>
+    </defs>
     <!-- Book body -->
     <path class="logo-book" d="M4 7 C4 7 8 5 16 6 C24 5 28 7 28 7 L28 26 C28 26 24 24 16 25 C8 24 4 26 4 26 Z" stroke-width="0.5"/>
     <!-- Book spine -->
@@ -37,7 +43,7 @@ defineProps<{ size?: number }>();
 
 <style scoped>
 .logo-book {
-  fill: var(--color-primary);
+  fill: url(#logo-gradient);
   stroke: color-mix(in srgb, var(--color-primary) 70%, transparent);
 }
 .logo-spine {