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/tui2/src/chatwidget.rs

@@ -1537,6 +1537,12 @@ impl ChatWidget {
         }
         match cmd {
             SlashCommand::Feedback => {
+                if !self.config.feedback_enabled {
+                    let params = crate::bottom_pane::feedback_disabled_params();
+                    self.bottom_pane.show_selection_view(params);
+                    self.request_redraw();
+                    return;
+                }
                 // Step 1: pick a category (UI built in feedback_view)
                 let params =
                     crate::bottom_pane::feedback_selection_params(self.app_event_tx.clone());