Add config to disable /feedback (#8909)

Some enterprises do not want their users to be able to `/feedback`.

<img width="395" height="325" alt="image"
src="https://github.com/user-attachments/assets/2dae9c0b-20c3-4a15-bcd3-0187857ebbd8"
/>

Adds to `config.toml`:

```toml
[feedback]
enabled = false
```

I've deliberately decided to:
1. leave other references to `/feedback` (e.g. in the interrupt message,
tips of the day) unchanged. I think we should continue to promote the
feature even if it is not usable currently.
2. leave the `/feedback` menu item selectable and display an error
saying it's disabled, rather than remove the menu item (which I believe
would raise more questions).

but happy to discuss these.

This will be followed by a change to requirements.toml that admins can
use to force the value of feedback.enabled.

codex-rs/tui/src/bottom_pane/feedback_view.rs

@@ -380,6 +380,21 @@ pub(crate) fn feedback_selection_params(
     }
 }
 
+/// Build the selection popup params shown when feedback is disabled.
+pub(crate) fn feedback_disabled_params() -> super::SelectionViewParams {
+    super::SelectionViewParams {
+        title: Some("Sending feedback is disabled".to_string()),
+        subtitle: Some("This action is disabled by configuration.".to_string()),
+        footer_hint: Some(standard_popup_hint_line()),
+        items: vec![super::SelectionItem {
+            name: "Close".to_string(),
+            dismiss_on_select: true,
+            ..Default::default()
+        }],
+        ..Default::default()
+    }
+}
+
 fn make_feedback_item(
     app_event_tx: AppEventSender,
     name: &str,