feat(smzdm): require 1:1 cover and surface image fetch failure prompt

Add SMZDM to the cover-required platform list on both client and server,
register a 800x800 native cover spec in the admin cover catalog, and
normalize the desktop-client `smzdm_image_fetch_failed` adapter error to a
user-friendly Chinese prompt across the publisher error pipeline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/media_service.go

@@ -151,7 +151,7 @@ func publishBatchRequiresCover(accounts []platformAccountSeed) bool {
 
 func publishPlatformRequiresCover(platformID string) bool {
 	switch strings.TrimSpace(platformID) {
-	case "weixin_gzh", "baijiahao", "dongchedi":
+	case "weixin_gzh", "baijiahao", "dongchedi", "smzdm":
 		return true
 	default:
 		return false