Add file tree to file view page (#32721)

Resolve #29328

This pull request introduces a file tree on the left side when reviewing
files of a repository.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/user/setting/settings.go

@@ -0,0 +1,26 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"net/http"
+	"strconv"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/json"
+	"code.gitea.io/gitea/services/context"
+)
+
+func UpdatePreferences(ctx *context.Context) {
+	type preferencesForm struct {
+		CodeViewShowFileTree bool `json:"codeViewShowFileTree"`
+	}
+	form := &preferencesForm{}
+	if err := json.NewDecoder(ctx.Req.Body).Decode(&form); err != nil {
+		ctx.HTTPError(http.StatusBadRequest, "json decode failed")
+		return
+	}
+	_ = user_model.SetUserSetting(ctx, ctx.Doer.ID, user_model.SettingsKeyCodeViewShowFileTree, strconv.FormatBool(form.CodeViewShowFileTree))
+	ctx.JSONOK()
+}