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/src/subtask-breakdown.ts

@@ -188,7 +188,7 @@ function persistSubtaskSession(session: SubtaskInternalSession, status: "generat
     status,
     title: session.initialDescription.slice(0, 120),
     inputPayload: JSON.stringify({ initialDescription: session.initialDescription }),
-    conversationHistory: "[]",
+    conversationHistory: JSON.stringify([{ thinkingOutput: session.thinkingOutput || "" }]),
     currentQuestion: null,
     result: session.subtasks.length > 0 ? JSON.stringify(session.subtasks) : null,
     thinkingOutput: session.thinkingOutput,