streamline ui (#1733)

Simplify and improve many UI elements.
* Remove all-around borders in most places. These interact badly with
terminal resizing and look heavy. Prefer left-side-only borders.
* Make the viewport adjust to the size of its contents.
* <kbd>/</kbd> and <kbd>@</kbd> autocomplete boxes appear below the
prompt, instead of above it.
* Restyle the keyboard shortcut hints & move them to the left.
* Restyle the approval dialog.
* Use synchronized rendering to avoid flashing during rerenders.


https://github.com/user-attachments/assets/96f044af-283b-411c-b7fc-5e6b8a433c20

<img width="1117" height="858" alt="Screenshot 2025-07-30 at 5 29 20 PM"
src="https://github.com/user-attachments/assets/0cc0af77-8396-429b-b6ee-9feaaccdbee7"
/>

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

@@ -33,6 +33,10 @@ impl BottomPaneView<'_> for StatusIndicatorView {
         true
     }
 
+    fn desired_height(&self, width: u16) -> u16 {
+        self.view.desired_height(width)
+    }
+
     fn render(&self, area: ratatui::layout::Rect, buf: &mut Buffer) {
         self.view.render_ref(area, buf);
     }