feat(FN-1151): preserve and display planning conversation history

- Persist per-turn thinking output for planning and mission interview sessions, including history serialization and recovery-safe state fields
- Add shared conversation history parsing/API types and a reusable timeline component with formatted answers and expandable AI reasoning
- Restore and render conversation history across Planning Mode, Mission Interview, and Subtask Breakdown modals during resume and live question flow
- Harden response submission with nullable-question guards and expand unit/e2e coverage for history rendering and persistence behavior

packages/dashboard/app/api.ts

@@ -3302,6 +3302,12 @@ export interface AiSessionSummary {
   updatedAt: string;
 }
 
+export interface ConversationHistoryEntry {
+  question?: PlanningQuestion;
+  response?: Record<string, unknown>;
+  thinkingOutput?: string;
+}
+
 export interface AiSessionDetail extends AiSessionSummary {
   inputPayload: string;
   conversationHistory: string;
@@ -3312,6 +3318,17 @@ export interface AiSessionDetail extends AiSessionSummary {
   createdAt: string;
 }
 
+export function parseConversationHistory(raw: string): ConversationHistoryEntry[] {
+  if (!raw) return [];
+
+  try {
+    const parsed = JSON.parse(raw);
+    return Array.isArray(parsed) ? parsed : [];
+  } catch {
+    return [];
+  }
+}
+
 export async function fetchAiSessions(projectId?: string): Promise<AiSessionSummary[]> {
   const params = projectId ? `?projectId=${encodeURIComponent(projectId)}` : "";
   const res = await fetch(buildApiUrl(`/ai-sessions${params}`));