fix: pass platform prop to GroupBadge in GroupSelector

GroupBadge in GroupSelector was missing the platform prop, causing all
group badges in account edit/detail pages to use fallback colors instead
of platform-specific colors (e.g. Claude=orange, Gemini=blue).

frontend/src/components/common/GroupSelector.vue

@@ -22,6 +22,7 @@
         />
         <GroupBadge
           :name="group.name"
+          :platform="group.platform"
           :subscription-type="group.subscription_type"
           :rate-multiplier="group.rate_multiplier"
           class="min-w-0 flex-1"