fix: remove favicon alt text to prevent Firefox flash in site picker

Firefox renders alt text while images load from Google's favicon service,
causing "Ci" to flash briefly in the 28px container before the icon appears.

components/dashboard/Sidebar.tsx

@@ -132,7 +132,7 @@ function SitePicker({ sites, siteId, collapsed, onExpand, onCollapse, wasCollaps
           {faviconUrl && !faviconFailed && (
             <img
               src={faviconUrl}
-              alt={currentSite?.name || ''}
+              alt=""
               className="w-5 h-5 object-contain"
               onError={() => setFaviconFailed(true)}
             />