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/slash_command.rs

@@ -15,6 +15,8 @@ pub enum SlashCommand {
     New,
     Diff,
     Quit,
+    #[cfg(debug_assertions)]
+    TestApproval,
 }
 
 impl SlashCommand {
@@ -26,6 +28,8 @@ impl SlashCommand {
             SlashCommand::Diff => {
                 "Show git diff of the working directory (including untracked files)"
             }
+            #[cfg(debug_assertions)]
+            SlashCommand::TestApproval => "Test approval request",
         }
     }