refactor(achievement): 稀有度配色收进 constants，文字色适配浅色模式

四份重复的 RARITY_COLOR / RARITY_LABEL 合并到 utils/constants。
拆成两套：边框色块继续用饱和的原色，文字改走 useRarityColor，
按明暗主题切换，两套都压到 4.5:1 以上——原色是照深色底调的，
白底上白金只有 1.7:1、黄金 2.2:1，卡片上的稀有度 tag 看不清。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/admin/achievement/components/AchievementModal.vue

@@ -7,6 +7,12 @@ import {
   type MetricOption,
 } from "admin/api"
 import AchievementIcon from "shared/components/AchievementIcon.vue"
+import { RARITY_LABEL } from "utils/constants"
+
+const rarityOptions = Object.entries(RARITY_LABEL).map(([value, label]) => ({
+  label,
+  value,
+}))
 
 const props = defineProps<{
   show: boolean
@@ -118,15 +124,7 @@ async function save() {
         </n-flex>
       </n-form-item>
       <n-form-item label="稀有度">
-        <n-select
-          v-model:value="form.rarity"
-          :options="[
-            { label: '青铜', value: 'bronze' },
-            { label: '白银', value: 'silver' },
-            { label: '黄金', value: 'gold' },
-            { label: '白金', value: 'platinum' },
-          ]"
-        />
+        <n-select v-model:value="form.rarity" :options="rarityOptions" />
       </n-form-item>
       <n-form-item label="指标" required>
         <n-select

src/admin/achievement/list.vue

@@ -7,6 +7,7 @@ import {
 } from "admin/api"
 import AchievementIcon from "shared/components/AchievementIcon.vue"
 import AchievementModal from "./components/AchievementModal.vue"
+import { RARITY_LABEL } from "utils/constants"
 
 const message = useMessage()
 const dialog = useDialog()
@@ -16,13 +17,6 @@ const loading = ref(false)
 const showModal = ref(false)
 const editing = ref<AdminAchievement | null>(null)
 
-const RARITY_LABEL: Record<string, string> = {
-  bronze: "青铜",
-  silver: "白银",
-  gold: "黄金",
-  platinum: "白金",
-}
-
 // 下架的只在有的时候才提，全部上架时标题不啰嗦
 const title = computed(() => {
   if (!list.value.length) return "成就管理"