Improve PageSpeed accessibility, SEO, and performance scores

Add alt attributes, width/height for CLS, aria-labels, meta description,
preconnect hints, LCP fetchpriority, and sound/stamp cache headers.

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

vue/src/components/util/Slideshow.vue

@@ -41,7 +41,7 @@ onUnmounted(() => {
                 <Header v-if="currentComment">
                     {{ currentComment }}
                 </Header>
-                <img :src="currentUrl" alt="Image Viewer" loading="lazy" />
+                <img :src="currentUrl" alt="Image Viewer" fetchpriority="high" />
             </div>
         </Transition>
     </div>