perf(images): add loading=lazy to in-list previews and attachments

Four image sites that render thumbnails/previews inside lists or tab
panels now hint loading=lazy so the browser defers the network fetch
until the element is near the viewport:
- Support.tsx attachment preview grid
- AdminTickets.tsx reply attachment grid
- BroadcastPreview.tsx photo preview
- BrandingTab.tsx logo preview in admin settings

Top-of-page logos (AppHeader/AppShell/DesktopSidebar/Login/Telegram
redirect/DeepLink) intentionally stay eager — they're above the fold
and should paint on first frame. Payment-method icons (6x6 px) are
already negligible. Compose attachment previews use blob: URLs where
loading=lazy is a no-op.

src/components/broadcasts/BroadcastPreview.tsx

@@ -217,7 +217,12 @@ export function TelegramPreview({
         <div className="rounded-xl bg-[#0e1621] p-3">
           <div className="ml-auto max-w-[90%] rounded-2xl rounded-tr-md bg-[#2b5278] p-3 text-white shadow">
             {mediaUrl && mediaType === 'photo' && (
-              <img src={mediaUrl} alt="" className="mb-2 max-h-72 w-full rounded-lg object-cover" />
+              <img
+                src={mediaUrl}
+                alt=""
+                loading="lazy"
+                className="mb-2 max-h-72 w-full rounded-lg object-cover"
+              />
             )}
             {mediaUrl && mediaType === 'video' && (
               <video src={mediaUrl} controls className="mb-2 max-h-72 w-full rounded-lg" />