feat: fullscreen image lightbox on image/diagram click

Click a note image or a mermaid/tikz diagram to view it fullscreen over
a blurred dim backdrop, with a fade/scale animation and rounded white
frame. Works on desktop and mobile; diagram SVGs are serialized to a
data URL via svgToDataUrl. Closes on click or Escape.

src/App.vue

@@ -1,4 +1,5 @@
 <script lang="ts" setup>
+import ImageLightbox from "@/components/ImageLightbox.vue"
 import NewVersion from "@/components/NewVersion.vue"
 import { useATProtoLogin } from "@/hooks/useATProtoLogin.hook"
 import { useGitHubLogin } from "@/hooks/useGitHubLogin.hook"
@@ -12,6 +13,7 @@ const { isATProtoReady } = useATProtoLogin()
     <router-view v-if="isReady && isATProtoReady" />
 
     <new-version />
+    <image-lightbox />
   </div>
 </template>