perf: bound SWR cache, clean stale storage, cap annotations

Add LRU cache provider (200 entries) to prevent unbounded SWR memory
growth. Clean up stale PKCE localStorage keys on app init. Cap chart
annotations to 20 visible reference lines with overflow indicator.

lib/utils/storage-cleanup.ts

@@ -0,0 +1,17 @@
+// * Cleans up stale localStorage entries on app initialization
+// * Prevents accumulation from abandoned OAuth flows
+
+export function cleanupStaleStorage() {
+  if (typeof window === 'undefined') return
+
+  try {
+    // * PKCE keys are only needed during the OAuth callback
+    // * If we're not on the callback page, they're stale leftovers
+    if (!window.location.pathname.includes('/auth/callback')) {
+      localStorage.removeItem('oauth_state')
+      localStorage.removeItem('oauth_code_verifier')
+    }
+  } catch {
+    // * Ignore errors (private browsing, storage disabled, etc.)
+  }
+}