Update tui error handling

- Allow each view to position error banners where they choose
- Add global 'esc' key handler to dismiss errors

pkg/tui/views/chat/chat.go

@@ -83,6 +83,7 @@ type Model struct {
 	stopSignal      chan struct{}
 	chatReplyChunks chan provider.Chunk
 	persistence     bool // whether we will save new messages in the conversation
+	promptCaching   bool // whether prompt caching is enabled
 
 	// UI state
 	focus          focusState