Add Fast mode status-line indicator (#13670)

Addresses feature request #13660

Adds new option to `/statusline` so the status line can display "fast
on" or "fast off"

Summary
- introduce a `FastMode` status-line item so `/statusline` can render
explicit `Fast on`/`Fast off` text for the service tier
- wire the item into the picker metadata and resolve its string from
`ChatWidget` without adding any unrelated `thread-name` logic or storage
changes
- ensure the refresh paths keep the cached footer in sync when the
service tier (fast mode) changes

Testing
- Manually tested

Here's what it looks like when enabled:

<img width="366" height="75" alt="image"
src="https://github.com/user-attachments/assets/7f992d2b-6dab-49ed-aa43-ad496f56f193"
/>

codex-rs/tui/src/chatwidget.rs

@@ -5348,6 +5348,13 @@ impl ChatWidget {
                 format_tokens_compact(self.status_line_total_usage().output_tokens)
             )),
             StatusLineItem::SessionId => self.thread_id.map(|id| id.to_string()),
+            StatusLineItem::FastMode => Some(
+                if matches!(self.config.service_tier, Some(ServiceTier::Fast)) {
+                    "Fast on".to_string()
+                } else {
+                    "Fast off".to_string()
+                },
+            ),
         }
     }