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/utils/constants.ts

@@ -1,4 +1,4 @@
-import type { SUBMISSION_RESULT } from "./types"
+import type { AchievementRarity, SUBMISSION_RESULT } from "./types"
 
 export enum SubmissionStatus {
   compile_error = -2,
@@ -276,6 +276,42 @@ export enum ChartType {
   Activity,
 }
 
+// 成就稀有度
+export const RARITY_LABEL: Record<AchievementRarity, string> = {
+  bronze: "青铜",
+  silver: "白银",
+  gold: "黄金",
+  platinum: "白金",
+}
+
+// 边框、色块用的原色：饱和度够高，明暗底上都醒目
+export const RARITY_COLOR: Record<AchievementRarity, string> = {
+  bronze: "#b87333",
+  silver: "#9fa6b2",
+  gold: "#e0a300",
+  platinum: "#7dd3fc",
+}
+
+// 文字要另配一套。上面那组是照深色底调的，搬到白底上白金只有 1.7:1、
+// 黄金 2.2:1，12px 的稀有度标签根本看不清。取色见 useRarityColor
+export const RARITY_TEXT_COLOR: Record<
+  "dark" | "light",
+  Record<AchievementRarity, string>
+> = {
+  dark: {
+    bronze: "#d18d4f",
+    silver: "#b6bcc7",
+    gold: "#e0a300",
+    platinum: "#7dd3fc",
+  },
+  light: {
+    bronze: "#9a5b25",
+    silver: "#6b7280",
+    gold: "#a37500",
+    platinum: "#0284c7",
+  },
+}
+
 // 时间范围配置
 export const DURATION_OPTIONS = [
   { label: "本节课内", value: "hours:1" },