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/status_indicator_widget.rs

@@ -73,6 +73,10 @@ impl StatusIndicatorWidget {
         }
     }
 
+    pub fn desired_height(&self, _width: u16) -> u16 {
+        1
+    }
+
     /// Update the line that is displayed in the widget.
     pub(crate) fn update_text(&mut self, text: String) {
         self.text = text.replace(['\n', '\r'], " ");
@@ -91,8 +95,8 @@ impl WidgetRef for StatusIndicatorWidget {
         let widget_style = Style::default();
         let block = Block::default()
             .padding(Padding::new(1, 0, 0, 0))
-            .borders(Borders::ALL)
-            .border_type(BorderType::Rounded)
+            .borders(Borders::LEFT)
+            .border_type(BorderType::QuadrantOutside)
             .border_style(widget_style.dim());
         // Animated 3‑dot pattern inside brackets. The *active* dot is bold
         // white, the others are dim.