Header: increase height and logo size for better visibility

- Nav padding: 0.5rem → 0.75rem vertical, 1.25rem → 1.5rem horizontal
- AppLogo size: 24px → 34px so the book detail is legible

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

frontend/src/components/AppHeader.vue

@@ -81,7 +81,7 @@ onUnmounted(() => document.removeEventListener("click", handleClickOutside));
     <nav class="nav">
       <!-- Left: brand -->
       <router-link to="/" class="nav-brand">
-        <AppLogo />
+        <AppLogo :size="34" />
         Fabled Assistant
       </router-link>
 
@@ -172,7 +172,7 @@ onUnmounted(() => document.removeEventListener("click", handleClickOutside));
   position: relative;
 }
 .nav {
-  padding: 0.5rem 1.25rem;
+  padding: 0.75rem 1.5rem;
   display: flex;
   align-items: center;
   justify-content: space-between;