feat: display missing books count badge on series covers

Show an orange badge with BookX icon on series covers when the Stripstream
API reports missing books in the collection. Also display a warning status
badge on the series detail page header.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/i18n/messages/en/common.json

@@ -263,6 +263,7 @@
       },
       "showMore": "Show more",
       "showLess": "Show less",
+      "missing": "{{count}} missing",
       "toggleSidebar": "Toggle sidebar",
       "toggleTheme": "Toggle theme"
     }

src/i18n/messages/fr/common.json

@@ -262,7 +262,8 @@
         "remove": "Retiré des favoris"
       },
       "showMore": "Voir plus",
-      "showLess": "Voir moins"
+      "showLess": "Voir moins",
+      "missing": "{{count}} manquant(s)"
     }
   },
   "books": {