Indicate form field readonly via background, fix RunUser config (#37175, #37180) (#37184)

Backport #37175 by @silverwind

The `Run As Username` field on the install page was a `readonly` input
that looked editable but wasn't, confusing users. Style `readonly`
inputs with a subtle background, matching other frameworks.

<img width="735" height="131" alt="image"
src="https://github.com/user-attachments/assets/cb76ce71-faab-4300-811e-e4c503b59f9a"
/>

Backport #37180

The comment "so just use current one if config says default" is not
right anymore: "git" isn't the "default" value of RunUser (Comment out
app.example.ini #15807). The RunUser's value is from current session's
username.

Fixes #37174

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/install/install.go

@@ -26,7 +26,6 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/timeutil"
-	"code.gitea.io/gitea/modules/user"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/routers/common"
@@ -87,15 +86,7 @@ func Install(ctx *context.Context) {
 	form.AppName = setting.AppName
 	form.RepoRootPath = setting.RepoRootPath
 	form.LFSRootPath = setting.LFS.Storage.Path
-
-	// Note(unknown): it's hard for Windows users change a running user,
-	// 	so just use current one if config says default.
-	if setting.IsWindows && setting.RunUser == "git" {
-		form.RunUser = user.CurrentUsername()
-	} else {
-		form.RunUser = setting.RunUser
-	}
-
+	form.RunUser = setting.RunUser
 	form.Domain = setting.Domain
 	form.SSHPort = setting.SSH.Port
 	form.HTTPPort = setting.HTTPPort
@@ -272,13 +263,6 @@ func SubmitInstall(ctx *context.Context) {
 		return
 	}
 
-	currentUser, match := setting.IsRunUserMatchCurrentUser(form.RunUser)
-	if !match {
-		ctx.Data["Err_RunUser"] = true
-		ctx.RenderWithErrDeprecated(ctx.Tr("install.run_user_not_match", form.RunUser, currentUser), tplInstall, &form)
-		return
-	}
-
 	// Check logic loophole between disable self-registration and no admin account.
 	if form.DisableRegistration && len(form.AdminName) == 0 {
 		ctx.Data["Err_Services"] = true