fix: 修复后端配置加载测试失败

- 修复 viper SafeWriteConfig 与 SetConfigFile 不兼容问题
  - 将 SafeWriteConfig() 替换为 SafeWriteConfigAs(configPath)
  - 绕过 viper 的 configPaths 检查
- 调整 Makefile 测试命令分类
  - backend-test: 仅运行后端核心测试
  - backend-test-all: 运行全部后端测试（含 desktop）
  - desktop-test: 单独运行桌面应用测试
- 同步 config-management 和 test-coverage 规范

Makefile

@@ -1,11 +1,11 @@
 .PHONY: all dev build test lint clean \
-	backend-build backend-run backend-dev backend-test backend-test-unit backend-test-integration backend-test-coverage \
+	backend-build backend-run backend-dev backend-test backend-test-all backend-test-unit backend-test-integration backend-test-coverage \
 	backend-lint backend-clean backend-deps backend-generate \
 	backend-db-up backend-db-down backend-db-status backend-db-create \
 	test-mysql-up test-mysql-down test-mysql test-mysql-quick \
 	frontend-build frontend-dev frontend-test frontend-test-watch frontend-test-coverage frontend-test-e2e frontend-lint frontend-clean \
 	desktop-build desktop-build-mac desktop-build-win desktop-build-linux \
-	desktop-dev desktop-package-mac desktop-package-win desktop-package-linux desktop-clean \
+	desktop-dev desktop-test desktop-package-mac desktop-package-win desktop-package-linux desktop-clean \
 	desktop-prepare-frontend desktop-prepare-embedfs
 
 # ============================================
@@ -19,7 +19,7 @@ dev:
 build: backend-build frontend-build
 	@echo "✅ Build complete"
 
-test: backend-test frontend-test
+test: backend-test desktop-test frontend-test
 	@echo "✅ All tests passed"
 
 lint: backend-lint frontend-lint
@@ -41,6 +41,9 @@ backend-dev:
 	cd backend && go run ./cmd/server
 
 backend-test:
+	cd backend && go test ./internal/... ./pkg/... ./tests/... ./cmd/server/... -v
+
+backend-test-all:
 	cd backend && go test ./... -v
 
 backend-test-unit:
@@ -179,6 +182,9 @@ desktop-dev: desktop-prepare-frontend desktop-prepare-embedfs
 	@echo "🖥️  Starting desktop app in dev mode..."
 	cd backend && go run ./cmd/desktop
 
+desktop-test:
+	cd backend && go test ./cmd/desktop/... -v
+
 desktop-package-mac:
 	./scripts/build/package-macos.sh