feat: copy tweaks (#2502)

- For selectable options, use sentences starting in lowercase and not
ending with periods. To be honest I don't love this style, but better to
be consistent for now.
- Tweak some other strings.
- Put in more compelling suggestions on launch. Excited to put `/mcp` in
there next.

codex-rs/tui/src/chatwidget.rs

@@ -771,7 +771,7 @@ impl ChatWidget<'_> {
         }
 
         self.bottom_pane.show_selection_view(
-            "Select Approvals Mode".to_string(),
+            "Select Approval Mode".to_string(),
             None,
             Some("Press Enter to confirm or Esc to go back".to_string()),
             items,