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>

web_src/css/modules/form.css

@@ -99,6 +99,13 @@ textarea:focus,
   color: var(--color-input-text);
 }
 
+.ui.form input:not([type="checkbox"], [type="radio"])[readonly],
+.ui.form textarea[readonly],
+.ui.form select[readonly],
+.ui.form .ui.selection.dropdown[readonly] {
+  background: var(--color-secondary-bg);
+}
+
 .ui.input {
   color: var(--color-input-text);
 }
@@ -198,7 +205,6 @@ textarea:focus,
   background-color: var(--color-error-bg);
   border-color: var(--color-error-border);
   color: var(--color-error-text);
-  border-radius: 0;
 }
 .ui.form .field.error textarea:focus,
 .ui.form .field.error select:focus,