refactor(assets): swap cosmic-stag PNG/WebP → AVIF

Replace the WebP cosmic-stag with an AVIF version that has cleaner
transparency. AVIF support is universal across the browsers we target
(Chrome ≥85, Firefox ≥93, Safari ≥16, Edge ≥121). Asset is ~92% the
size of the WebP it replaces.

Updates the four import sites: index.html favicon, SiteHeader logo,
SiteFooter logo and the HomeView hero stag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/SiteFooter.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { useI18n } from 'vue-i18n'
-import cosmicStag from '@/assets/cosmic-stag.webp'
+import cosmicStag from '@/assets/cosmic-stag.avif'
 
 const { t } = useI18n()
 const year = new Date().getFullYear()

src/components/SiteHeader.vue

@@ -2,7 +2,7 @@
 import { ref, computed, onMounted, onUnmounted, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { RouterLink, useRoute } from 'vue-router'
-import cosmicStag from '@/assets/cosmic-stag.webp'
+import cosmicStag from '@/assets/cosmic-stag.avif'
 
 const { t, locale } = useI18n()
 const route = useRoute()

src/views/HomeView.vue

@@ -2,7 +2,7 @@
 import { useI18n } from 'vue-i18n'
 import { RouterLink } from 'vue-router'
 import { Button } from '@/components/ui/button'
-import cosmicStag from '@/assets/cosmic-stag.webp'
+import cosmicStag from '@/assets/cosmic-stag.avif'
 import heroLandscape from '@/assets/hero-landscape.webp'
 
 const { t } = useI18n()