feat: use app icon for sidebar brand mark

Replace 'iT' text badge with actual icon image.

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

imptune/templates/base.html

@@ -432,7 +432,7 @@
     <div class="nav-scrim" x-show="nav" @click="nav = false" style="display:none"></div>
     <nav class="sidebar" @click="nav = false">
       <a class="brand" href="/">
-        <span class="brand-mark">iT</span>
+        <img class="brand-mark" src="/static/favicon.png" alt="ImpTune">
         <span>
           <span class="brand-name">ImpTune</span>
           <span class="brand-sub" x-data x-text="$store.i18n.t('brand_sub')">Print packaging</span>