fix: use VitePress withBase in NavCard for correct routing

NavCard component now uses VitePress's withBase() utility function
to properly handle base path in all environments:
- Local development: /easy-vibe/
- GitHub Pages: /easy-vibe/
- Vercel: /

This fixes 404 errors when clicking NavCard links in appendix pages.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/.vitepress/theme/components/NavCard.vue

@@ -1,4 +1,6 @@
 <script setup>
+import { withBase } from 'vitepress'
+
 defineProps({
   href: {
     type: String,
@@ -21,7 +23,7 @@ defineProps({
 
 <template>
   <a
-    :href="href"
+    :href="withBase(href)"
     class="nav-card-link"
   >
     <div class="nav-card">