Keep file tree view icons consistent with icon theme (#33921)

Fix #33914

before:
![3000-gogitea-gitea-y4ulxr46c4k ws-us118 gitpod io_test_test
gitea_src_branch_main_
gitmodules](https://github.com/user-attachments/assets/ca50eeff-cc44-4041-b01f-c0c5bdd3b6aa)

after:
![3000-gogitea-gitea-y4ulxr46c4k ws-us118 gitpod io_test_test
gitea_src_branch_main_README
md](https://github.com/user-attachments/assets/3b87fdbd-81d0-4831-8a74-4dbfcd5b6d91)

---------

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

modules/fileicon/render.go

@@ -0,0 +1,52 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package fileicon
+
+import (
+	"html/template"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+type RenderedIconPool struct {
+	IconSVGs map[string]template.HTML
+}
+
+func NewRenderedIconPool() *RenderedIconPool {
+	return &RenderedIconPool{
+		IconSVGs: make(map[string]template.HTML),
+	}
+}
+
+func (p *RenderedIconPool) RenderToHTML() template.HTML {
+	if len(p.IconSVGs) == 0 {
+		return ""
+	}
+	sb := &strings.Builder{}
+	sb.WriteString(`<div class=tw-hidden>`)
+	for _, icon := range p.IconSVGs {
+		sb.WriteString(string(icon))
+	}
+	sb.WriteString(`</div>`)
+	return template.HTML(sb.String())
+}
+
+// TODO: use an interface or struct to replace "*git.TreeEntry", to decouple the fileicon module from git module
+
+func RenderEntryIcon(renderedIconPool *RenderedIconPool, entry *git.TreeEntry) template.HTML {
+	if setting.UI.FileIconTheme == "material" {
+		return DefaultMaterialIconProvider().FileIcon(renderedIconPool, entry)
+	}
+	return BasicThemeIcon(entry)
+}
+
+func RenderEntryIconOpen(renderedIconPool *RenderedIconPool, entry *git.TreeEntry) template.HTML {
+	// TODO: add "open icon" support
+	if setting.UI.FileIconTheme == "material" {
+		return DefaultMaterialIconProvider().FileIcon(renderedIconPool, entry)
+	}
+	return BasicThemeIcon(entry)
+}