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/mod.rs

@@ -64,8 +64,11 @@ impl BottomPane<'_> {
         }
     }
 
-    pub fn desired_height(&self) -> u16 {
-        self.composer.desired_height()
+    pub fn desired_height(&self, width: u16) -> u16 {
+        self.active_view
+            .as_ref()
+            .map(|v| v.desired_height(width))
+            .unwrap_or(self.composer.desired_height())
     }
 
     /// Forward a key event to the active view or the composer.