feat(perf): performance composables for lazy loading, dedup, prefetch, cleanup (M18.1-M18.8)

- Image lazy loading: IntersectionObserver + blur-up placeholder
- Request deduplication: in-flight Promise sharing by URL+body key
- Prefetch on hover: 5-minute cache for pre-fetched detail data
- Memory leak audit: useCleanup() tracks intervals/listeners/observers
- Background sync queue: retry failed IDB saves on visibility change
- Bundle splitting: composable architecture enables tree-shaking

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

packages/app/src/composables/usePrefetch.ts

@@ -0,0 +1,32 @@
+const prefetchCache = new Map<string, { data: unknown; expiresAt: number }>()
+const CACHE_DURATION = 5 * 60 * 1000 // 5 minutes
+
+export function usePrefetch() {
+  function getCached<T>(key: string): T | null {
+    const entry = prefetchCache.get(key)
+    if (!entry) return null
+    if (Date.now() > entry.expiresAt) {
+      prefetchCache.delete(key)
+      return null
+    }
+    return entry.data as T
+  }
+
+  function setCached(key: string, data: unknown) {
+    prefetchCache.set(key, { data, expiresAt: Date.now() + CACHE_DURATION })
+  }
+
+  async function prefetch(key: string, fetcher: () => Promise<unknown>) {
+    if (getCached(key)) return
+    try {
+      const data = await fetcher()
+      setCached(key, data)
+    } catch { /* ignore prefetch errors */ }
+  }
+
+  function clearCache() {
+    prefetchCache.clear()
+  }
+
+  return { getCached, setCached, prefetch, clearCache }
+}