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/admin/BrandingTab.tsx

@@ -116,6 +116,7 @@ export function BrandingTab({ accentColor = '#3b82f6' }: BrandingTabProps) {
                 <img
                   src={brandingApi.getLogoUrl(branding) ?? undefined}
                   alt="Logo"
+                  loading="lazy"
                   className="h-full w-full object-cover"
                 />
               ) : (