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>

custom/conf/app.example.ini

@@ -41,10 +41,10 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; App name that shows in every page title
-APP_NAME = ; Gitea: Git with a cup of tea
+;APP_NAME = Gitea: Git with a cup of tea
 ;;
 ;; RUN_USER will automatically detect the current user - but you can set it here change it if you run locally
-RUN_USER = ; git
+;RUN_USER =
 ;;
 ;; Application run mode, affects performance and debugging: "dev" or "prod", default is "prod"
 ;; Mode "dev" makes Gitea easier to develop and debug, values other than "dev" are treated as "prod" which is for production use.