fix(update): increase npm registry fetch timeout from 5s to 15s (#1724)

The update checker fetches https://registry.npmjs.org/@larksuite/cli/latest
with a 5-second HTTP client timeout. Under high-latency network conditions
(TUN-mode proxies, VPNs, transcontinental routes), TLS handshake alone can
take 4-6 seconds, causing the check to fail with:

  context deadline exceeded (Client.Timeout exceeded while awaiting headers)

Measured example behind a Clash TUN proxy (US node from China):
  DNS resolve:    ~0ms (fake-ip)
  TCP connect:    ~0ms (local TUN)
  TLS handshake:  4.3-5.9s  <-- bottleneck
  Total:          4.7-6.3s

curl succeeds because it has no default connect timeout, but the Go HTTP
client with Timeout=5s is too tight. The registry endpoint returns a tiny
JSON payload (<1KB), so 15s is more than enough headroom while still
failing fast on genuinely unreachable networks.

Co-authored-by: 王伟达 <weida.wang@m.com>

internal/update/update.go

@@ -25,7 +25,7 @@ import (
 const (
 	registryURL  = "https://registry.npmjs.org/@larksuite/cli/latest"
 	cacheTTL     = 24 * time.Hour
-	fetchTimeout = 5 * time.Second
+	fetchTimeout = 15 * time.Second
 	stateFile    = "update-state.json"
 	maxBody      = 256 << 10 // 256 KB