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/static/app.css

@@ -316,16 +316,8 @@ nav.sidebar li {
 .brand-mark {
   width: 26px;
   height: 26px;
-  border-radius: 6px;
-  background: var(--im-accent);
-  color: var(--pico-primary-inverse);
-  display: grid;
-  place-items: center;
-  font-family: var(--im-mono);
-  font-size: .75rem;
-  font-weight: 700;
-  letter-spacing: -.03em;
   flex: 0 0 auto;
+  object-fit: contain;
 }
 
 .brand-name {

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>