fix(proxy): encode special chars in proxy credentials

backend/internal/service/proxy.go

 package service
 
 import (
-	"fmt"
+	"net"
+	"net/url"
+	"strconv"
 	"time"
 )
 
@@ -23,10 +25,14 @@ func (p *Proxy) IsActive() bool {
 }
 
 func (p *Proxy) URL() string {
+	u := &url.URL{
+		Scheme: p.Protocol,
+		Host:   net.JoinHostPort(p.Host, strconv.Itoa(p.Port)),
+	}
 	if p.Username != "" && p.Password != "" {
-		return fmt.Sprintf("%s://%s:%s@%s:%d", p.Protocol, p.Username, p.Password, p.Host, p.Port)
+		u.User = url.UserPassword(p.Username, p.Password)
 	}
-	return fmt.Sprintf("%s://%s:%d", p.Protocol, p.Host, p.Port)
+	return u.String()
 }
 
 type ProxyWithAccountCount struct {

backend/internal/service/proxy_test.go

+package service
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestProxyURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		proxy Proxy
+		want  string
+	}{
+		{
+			name: "without auth",
+			proxy: Proxy{
+				Protocol: "http",
+				Host:     "proxy.example.com",
+				Port:     8080,
+			},
+			want: "http://proxy.example.com:8080",
+		},
+		{
+			name: "with auth",
+			proxy: Proxy{
+				Protocol: "socks5",
+				Host:     "socks.example.com",
+				Port:     1080,
+				Username: "user",
+				Password: "pass",
+			},
+			want: "socks5://user:pass@socks.example.com:1080",
+		},
+		{
+			name: "username only keeps no auth for compatibility",
+			proxy: Proxy{
+				Protocol: "http",
+				Host:     "proxy.example.com",
+				Port:     8080,
+				Username: "user-only",
+			},
+			want: "http://proxy.example.com:8080",
+		},
+		{
+			name: "with special characters in credentials",
+			proxy: Proxy{
+				Protocol: "http",
+				Host:     "proxy.example.com",
+				Port:     3128,
+				Username: "first last@corp",
+				Password: "p@ ss:#word",
+			},
+			want: "http://first%20last%40corp:p%40%20ss%3A%23word@proxy.example.com:3128",
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if got := tc.proxy.URL(); got != tc.want {
+				t.Fatalf("Proxy.URL() mismatch: got=%q want=%q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestProxyURL_SpecialCharactersRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	proxy := Proxy{
+		Protocol: "http",
+		Host:     "proxy.example.com",
+		Port:     3128,
+		Username: "first last@corp",
+		Password: "p@ ss:#word",
+	}
+
+	parsed, err := url.Parse(proxy.URL())
+	if err != nil {
+		t.Fatalf("parse proxy URL failed: %v", err)
+	}
+	if got := parsed.User.Username(); got != proxy.Username {
+		t.Fatalf("username mismatch after parse: got=%q want=%q", got, proxy.Username)
+	}
+	pass, ok := parsed.User.Password()
+	if !ok {
+		t.Fatal("password missing after parse")
+	}
+	if pass != proxy.Password {
+		t.Fatalf("password mismatch after parse: got=%q want=%q", pass, proxy.Password)
+	}
+}