perf(app): add 30s cache TTL to Bitcoin price fetcher

Skips redundant API calls when price was fetched within the last 30
seconds, reducing network requests while keeping data fresh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/composables/useBitcoinPrice.ts

@@ -8,7 +8,12 @@ const isLoading = ref(false)
 let refreshTimer: ReturnType<typeof setInterval> | null = null
 let initialized = false
 
+const CACHE_TTL = 30_000 // 30 seconds
+
 async function fetchPrice() {
+  // Skip if we fetched recently (within TTL)
+  if (lastUpdated.value && Date.now() - lastUpdated.value < CACHE_TTL) return
+
   isLoading.value = true
   try {
     const res = await fetch('https://mempool.space/api/v1/prices')