fix(usage): 移动端缓存 token 拆分为读/写两列展示

cache_read 与 cache_creation 原本被合并为单值，现分别显示并补充测试

frontend/src/features/usage/token-normalization.ts

@@ -30,6 +30,10 @@ export function getCacheCreationTokens(usage: UsageTokenLike): number {
   return explicit
 }
 
+export function getCacheReadTokens(usage: UsageTokenLike): number {
+  return toNonNegativeNumber(usage.cache_read_input_tokens)
+}
+
 export function getEffectiveInputTokens(usage: UsageTokenLike): number {
   const explicit = toNonNegativeNumber(usage.effective_input_tokens)
   if (explicit > 0) {