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>

modules/setting/setting.go

@@ -201,7 +201,7 @@ func mustCurrentRunUserMatch(rootCfg ConfigProvider) {
 	if HasInstallLock(rootCfg) {
 		currentUser, match := IsRunUserMatchCurrentUser(RunUser)
 		if !match {
-			log.Fatal("Expect user '%s' but current user is: %s", RunUser, currentUser)
+			log.Fatal("Expect user '%s' (RUN_USER in app.ini) but current user is: %s", RunUser, currentUser)
 		}
 	}
 }