Cache message history locally

[ih8js-git]

Problem Description

Currently, vimstoat only caches User objects in the local cache.db (CacheStore). Message history is not cached anywhere on disk:

1. Opening a DM channel triggers a fresh network call to fetch up to 50 messages via fetch_message_history.
2. Loaded messages and live WebSocket message events (new messages, edits, deletes) are stored solely in memory in app.store.current_dm_messages: Vec<Message>.
3. Navigating between channels or restarting the app discards message history, requiring full re-fetching over the network each time.

Proposed Solution

• Persist messages locally (e.g. keyed by channel_id + message ID or in an indexed structure per channel in CacheStore).
• Hydrate channel messages instantly from local cache when entering a channel.
• Fetch newer/missing messages from the API to sync and update the cache.
• Ensure WebSocket updates (edits, deletions) write through or invalidate the cache accordingly.