feat: 统一品牌标识、关于页面三卡片布局与版本诊断功能

- 统一品牌为 Nex：侧边栏、托盘 tooltip、HTML 标题、favicon (PNG 替代 SVG)
- 重构关于页面为三卡片布局（品牌/版本/链接），版本状态 Tag 绝对定位右上角
- 新增 GET /api/version 后端接口，返回 version/commit/build_time
- 新增前端版本一致性诊断：匹配/不匹配/不可判断三种状态
- 同步 delta specs 到主 specs 并归档变更

backend/internal/handler/version_handler_test.go

@@ -0,0 +1,31 @@
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestVersionHandler_GetVersion(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	h := NewVersionHandler()
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/api/version", nil)
+
+	h.GetVersion(c)
+
+	assert.Equal(t, http.StatusOK, w.Code)
+
+	var result map[string]string
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "dev", result["version"])
+	assert.Equal(t, "unknown", result["commit"])
+	assert.Equal(t, "unknown", result["build_time"])
+}