fix: make BackButton and LinkedNotes keyboard accessible

Replace <a> (no href) with <button> so both elements receive tab focus.
BackButton gets text-base-content to preserve icon color; LinkedNotes
uses btn class="link" to keep the inline text-link appearance.

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

src/components/BackButton.vue

@@ -24,7 +24,7 @@ const goBack = () => {
 </script>
 
 <template>
-  <a class="btn btn-sm back-button" @click="goBack">
+  <button class="btn btn-sm back-button text-base-content" @click="goBack">
     <svg
       xmlns="http://www.w3.org/2000/svg"
       class="icon icon-tabler icon-tabler-arrow-narrow-left"
@@ -41,5 +41,5 @@ const goBack = () => {
       <line x1="5" y1="12" x2="9" y2="16" />
       <line x1="5" y1="12" x2="9" y2="8" />
     </svg>
-  </a>
+  </button>
 </template>

src/components/LinkedNotes.vue

@@ -24,9 +24,9 @@ const emitNote = (sha: string) => {
     <h5 class="subtitle is-5">🔗</h5>
     <ul class="links">
       <li v-for="link in backlink?.links" :key="link.sha">
-        <a @click.prevent="emitNote(link.sha)">
+        <button class="link" @click="emitNote(link.sha)">
           {{ link.title }}
-        </a>
+        </button>
       </li>
     </ul>
   </div>