refactor(chat): split ChatScreen and reuse regenerateResponse after editing a message

The edit path duplicated the streaming flow with inline require() calls; it now reuses regenerateResponse, which also fixes:
- edited conversations were never saved to History
- the answer generated after an edit could not be stopped (no abort handle kept)
Stopping a stream now saves the partial conversation, like finishing or failing does.
Adds characterization tests for ChatScreen and useLMStudio.

utils/chatSelectors.ts

@@ -0,0 +1,16 @@
+import type { ChatMessage } from '../store/useAppStore';
+
+/** Id of the most recent assistant message, or null when there is none. */
+export function findLastAssistantId(messages: ChatMessage[]): string | null {
+    for (let i = messages.length - 1; i >= 0; i--) {
+        if (messages[i].role === 'assistant') return messages[i].id;
+    }
+    return null;
+}
+
+/** True while the model has been asked to answer but has not produced any text yet. */
+export function shouldShowTypingIndicator(messages: ChatMessage[], isStreaming: boolean): boolean {
+    if (!isStreaming) return false;
+    const last = messages[messages.length - 1];
+    return last?.role === 'assistant' && last.content === '';
+}