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>

options/locale/locale_en-US.json

@@ -269,7 +269,7 @@
   "install.lfs_path": "Git LFS Root Path",
   "install.lfs_path_helper": "Files tracked by Git LFS will be stored in this directory. Leave empty to disable.",
   "install.run_user": "Run As Username",
-  "install.run_user_helper": "The operating system username that Gitea runs as. Note that this user must have access to the repository root path.",
+  "install.run_user_helper": "The operating system username that Gitea runs as, it must have write access to the data paths. This value is auto-detected and cannot be changed here. To use a different user, restart Gitea under that account.",
   "install.domain": "Server Domain",
   "install.domain_helper": "Domain or host address for the server.",
   "install.ssh_port": "SSH Server Port",
@@ -316,7 +316,6 @@
   "install.invalid_db_table": "The database table \"%s\" is invalid: %v",
   "install.invalid_repo_path": "The repository root path is invalid: %v",
   "install.invalid_app_data_path": "The app data path is invalid: %v",
-  "install.run_user_not_match": "The 'run as' username is not the current username: %s -> %s",
   "install.internal_token_failed": "Failed to generate internal token: %v",
   "install.secret_key_failed": "Failed to generate secret key: %v",
   "install.save_config_failed": "Failed to save configuration: %v",