tweak custom emoji handling

Close #9721

packages/frontend/src/components/MkReactionIcon.vue

@@ -1,5 +1,6 @@
 <template>
-<MkEmoji :emoji="reaction" :is-reaction="true" :normal="true" :no-style="noStyle"/>
+<MkCustomEmoji v-if="reaction[0] === ':'" :name="reaction" :normal="true" :no-style="noStyle" :url="emojiUrl"/>
+<MkEmoji v-else :emoji="reaction" :normal="true" :no-style="noStyle"/>
 </template>
 
 <script lang="ts" setup>
@@ -8,5 +9,6 @@ import { } from 'vue';
 const props = defineProps<{
 	reaction: string;
 	noStyle?: boolean;
+	emojiUrl?: string;
 }>();
 </script>