fix(ui): preserve prompt text when dismissing mention picker (#285)

## Summary
- preserve the current prompt text when dismissing the `@` mention/file
picker with `Esc`
- let `Enter` fall back to normal prompt submission when the mention
picker is open but there is no selectable result

## Verification
- source inspection of the prompt input and picker flow
- local `npm run typecheck --workspace @codenomad/ui` is blocked in this
environment because workspace dependencies are not installed

--
Yours,
[CodeNomadBot](https://github.com/NeuralNomadsAI/CodeNomad)

Co-authored-by: Shantur Rathore <i@shantur.com>

packages/ui/src/components/prompt-input.tsx

@@ -540,6 +540,10 @@ export default function PromptInput(props: PromptInputProps) {
               mode={pickerMode()}
               onClose={handlePickerClose}
               onSelect={handlePickerSelect}
+              onSubmitWithoutSelection={() => {
+                handlePickerClose()
+                void handleSend()
+              }}
               agents={instanceAgents()}
               commands={getCommands(props.instanceId)}
               instanceClient={instance()!.client}