fix: add fallback DNS resolver for Android with multi-DNS support (#1835)

On Android, /etc/resolv.conf does not exist, causing Go's default DNS
resolution to fail. This adds an init() hook that:

1. Detects missing /etc/resolv.conf (Android environment)
2. Configures a custom resolver with PreferGo: true
3. Supports multiple DNS servers via PICOCLAW_DNS_SERVER env var
   - Semicolon-separated: "8.8.8.8:53;1.1.1.1:53"
   - Single server also works: "8.8.8.8"
   - Auto-appends :53 if port omitted
4. Round-robin rotation across configured servers
5. Defaults to Google DNS + Cloudflare DNS

Also patches http.DefaultTransport to use the custom resolver.

cmd/picoclaw/dns_noresolv.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"os"
+	"strings"
+	"sync/atomic"
+	"time"
+)
+
+func init() {
+	// 仅在 /etc/resolv.conf 不存在时才覆盖（即 Android 环境）
+	if _, err := os.Stat("/etc/resolv.conf"); err == nil {
+		return
+	}
+
+	// 从环境变量获取 DNS server 列表，多个用 ; 隔开
+	// 例如: PICOCLAW_DNS_SERVER="8.8.8.8:53;1.1.1.1:53;223.5.5.5:53"
+	dnsEnv := os.Getenv("PICOCLAW_DNS_SERVER")
+	if dnsEnv == "" {
+		dnsEnv = "8.8.8.8:53;1.1.1.1:53"
+	}
+
+	var dnsServers []string
+	for _, s := range strings.Split(dnsEnv, ";") {
+		s = strings.TrimSpace(s)
+		if s != "" {
+			// 如果没有带端口号，自动补上 :53
+			if _, _, err := net.SplitHostPort(s); err != nil {
+				s = s + ":53"
+			}
+			dnsServers = append(dnsServers, s)
+		}
+	}
+
+	// 轮询索引，在多个 DNS 服务器之间轮转
+	var idx uint64
+
+	customResolver := &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+			d := net.Dialer{Timeout: 5 * time.Second}
+			// Round-robin: 依次尝试不同的 DNS 服务器
+			server := dnsServers[atomic.AddUint64(&idx, 1)%uint64(len(dnsServers))]
+			return d.DialContext(ctx, "udp", server)
+		},
+	}
+
+	// 覆盖全局 DefaultResolver
+	net.DefaultResolver = customResolver
+
+	// 覆盖 http.DefaultTransport 使用自定义 DNS 解析的 DialContext
+	dialer := &net.Dialer{
+		Timeout:   30 * time.Second,
+		KeepAlive: 30 * time.Second,
+		Resolver:  customResolver,
+	}
+
+	if tr, ok := http.DefaultTransport.(*http.Transport); ok {
+		tr.DialContext = dialer.DialContext
+	}
+}