Mobile: keep messages and recent threads available through connection drops

[aneym]

People use T3 Code on phones while traveling and in areas with weak coverage. A message sent during a connection gap can disappear or leave the composer blocked. Threads that were not opened recently are unavailable offline, and the reconnect state covers the thread.

I propose one client-only change for this workflow.

• Save each queued message before clearing the composer. Send messages in order after reconnecting. Reuse each commandId so existing server receipts deduplicate retries after an app restart or mid-send disconnect.
• Cache thread snapshots within a 150 MB LRU budget. Protect the open thread and threads with queued messages from eviction.
• While connected, prefetch recent threads two at a time through the existing afterSequence catch-up path. Start this work when the app enters the foreground and after reconnecting.
• Replace the reconnect overlay with a small delayed chip. Keep the thread readable and show the number of queued sends.

This proposal changes no server code or contracts. Background URLSession transfers and BGAppRefresh are out of scope.

A working implementation and simulator video are available at https://github.com/user-attachments/assets/9680d28e-9659-4cc4-873d-4a090c86083a. They demonstrate the proposal, not an expectation that maintainers accept it.

Would you approve this direction and scope for a PR, or would you prefer a narrower approach?

[aneym]

The implementation is up for review in #26013.