feat: agent proactively re-engages users on returning sessions

Two complementary changes to reduce conversation cold-start friction:

A. Returning-user re-greeting (backend)
- When WS reconnects to a non-empty conversation and last_message_at is older
  than chat_re_greeting_idle_hours (default 6h), the agent emits a warm
  continuation message that references prior history instead of staying silent.
- Self-debouncing: the AI message updates last_message_at, so reconnects
  within the window will not re-trigger.
- Skipped while profile collection is still pending.

D. Topic suggestion chips (backend + Expo)
- New WS message type topic_suggestions carries 3-4 quick-start chips derived
  from the current memoir stage's empty slots (deterministic, no extra LLM
  cost). Sent alongside opening / re-greeting / resume.
- Expo chat screen renders a horizontally-scrollable chip row above the input
  bar; tapping a chip sends the chip's text as a user message and clears the
  row. Sending any text/voice also clears the chips.

app-expo/src/i18n/locales/en/conversation.json

@@ -37,6 +37,7 @@
   "switchToVoice": "Switch to voice input",
   "tapToEndRecording": "Tap to end",
   "tapToStartRecording": "Tap to start recording",
+  "topicSuggestionsDismiss": "Hide",
   "viewAll": "View All",
   "voiceMessagePreview": "Voice message",
   "timeJustNow": "Just now",