feat: 区分 server 与 desktop 配置加载入口，取消自动创建配置文件

- config.go 重构：抽取 loadConfig 共享逻辑，新增 LoadServerConfig/LoadDesktopConfig/LoadDesktopConfigAtPath，LoadConfig 保持向后兼容
- setupConfigFile 移除 SafeWriteConfigAs 自动创建逻辑，文件不存在时仅使用默认值
- cmd/desktop 切换为 LoadDesktopConfig，端口/HTTP/浏览器/托盘统一使用 cfg.Server.Port
- cmd/server 显式使用 LoadServerConfig 明确入口语义
- 提取 desktop 可测 helper：desktopListenAddr/desktopURL/desktopPortMenuTitle/desktopConfigErrorMessage
- 新增测试：desktop 忽略 CLI/env/未知参数、配置快照不变、无效配置文件不静默回退、端口 helper 一致性
- README 区分 server/desktop 配置源，移除首次启动自动创建配置文件描述
- 同步 delta specs 到 openspec/specs/ 主规范

backend/tests/config/config_test.go

@@ -120,7 +120,7 @@ log:
 	assert.Equal(t, "warn", cfg.Log.Level, "YAML value should be used when no CLI/ENV override")
 }
 
-func TestLoadConfig_AutoCreate(t *testing.T) {
+func TestLoadConfig_NoAutoCreate(t *testing.T) {
 	tmpDir := t.TempDir()
 	configPath := filepath.Join(tmpDir, "config.yaml")
 
@@ -132,6 +132,9 @@ func TestLoadConfig_AutoCreate(t *testing.T) {
 	require.NotNil(t, cfg)
 
 	assert.Equal(t, 9826, cfg.Server.Port, "should load with default values")
+
+	_, err = os.Stat(configPath)
+	assert.True(t, os.IsNotExist(err), "config file should not be auto-created")
 }
 
 func TestSaveAndLoadConfig(t *testing.T) {
@@ -184,3 +187,124 @@ func TestSaveAndLoadConfig(t *testing.T) {
 	assert.Equal(t, cfg.Log.MaxAge, loaded.Log.MaxAge)
 	assert.Equal(t, cfg.Log.Compress, loaded.Log.Compress)
 }
+
+func TestLoadDesktopConfig_FileOnly(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config.yaml")
+
+	yamlContent := `
+server:
+  port: 8080
+log:
+  level: debug
+`
+	err := os.WriteFile(configPath, []byte(yamlContent), 0o600)
+	require.NoError(t, err)
+
+	cfg, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err)
+
+	assert.Equal(t, 8080, cfg.Server.Port)
+	assert.Equal(t, "debug", cfg.Log.Level)
+}
+
+func TestLoadDesktopConfig_IgnoresCLI(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config.yaml")
+
+	err := os.WriteFile(configPath, []byte("server:\n  port: 8080\n"), 0o600)
+	require.NoError(t, err)
+
+	originalArgs := os.Args
+	defer func() { os.Args = originalArgs }()
+	os.Args = []string{"nex", "--server-port", "9999"}
+
+	cfg, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err)
+
+	assert.Equal(t, 8080, cfg.Server.Port, "desktop should ignore CLI args and use config file")
+}
+
+func TestLoadDesktopConfig_IgnoresEnv(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config.yaml")
+
+	err := os.WriteFile(configPath, []byte("server:\n  port: 8080\n"), 0o600)
+	require.NoError(t, err)
+
+	t.Setenv("NEX_SERVER_PORT", "9000")
+	t.Setenv("NEX_LOG_LEVEL", "debug")
+
+	cfg, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err)
+
+	assert.Equal(t, 8080, cfg.Server.Port, "desktop should ignore env vars and use config file")
+	assert.Equal(t, "info", cfg.Log.Level, "desktop should ignore env vars and use default")
+}
+
+func TestLoadDesktopConfig_IgnoresUnknownArgs(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config.yaml")
+
+	err := os.WriteFile(configPath, []byte("server:\n  port: 8080\n"), 0o600)
+	require.NoError(t, err)
+
+	originalArgs := os.Args
+	defer func() { os.Args = originalArgs }()
+	os.Args = []string{"nex", "--unknown-flag", "value", "--another-unknown"}
+
+	cfg, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err, "desktop should not fail on unknown CLI args")
+
+	assert.Equal(t, 8080, cfg.Server.Port)
+}
+
+func TestLoadDesktopConfig_Snapshot(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config.yaml")
+
+	err := os.WriteFile(configPath, []byte("server:\n  port: 8080\n"), 0o600)
+	require.NoError(t, err)
+
+	cfg, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err)
+	assert.Equal(t, 8080, cfg.Server.Port)
+
+	err = os.WriteFile(configPath, []byte("server:\n  port: 9999\n"), 0o600)
+	require.NoError(t, err)
+
+	assert.Equal(t, 8080, cfg.Server.Port, "loaded config snapshot should not change when file changes")
+
+	cfg2, err := config.LoadDesktopConfigAtPath(configPath)
+	require.NoError(t, err)
+	assert.Equal(t, 9999, cfg2.Server.Port, "reload should pick up new config values")
+}
+
+func TestLoadDesktopConfig_InvalidFileFails(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+	}{
+		{
+			name:    "invalid yaml",
+			content: "server:\n  port: [\n",
+		},
+		{
+			name:    "validation failure",
+			content: "server:\n  port: 70000\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			configPath := filepath.Join(tmpDir, "config.yaml")
+
+			err := os.WriteFile(configPath, []byte(tt.content), 0o600)
+			require.NoError(t, err)
+
+			_, err = config.LoadDesktopConfigAtPath(configPath)
+			require.Error(t, err, "desktop should not silently fall back to defaults for invalid config files")
+		})
+	}
+}