feat: improve chat history paging and unread navigation

[shakkernerd]

Summary

• add cursor-based message history paging for channel and DM history
• keep the chat message window bounded while preserving unread, pending, selected, and scroll-anchor messages
• improve unread divider/bar behavior during history jumps and scroll-to-present flows
• add CI coverage for bot auth and paging paths so the current Go coverage gate stays green

Verification

• pnpm check
• pnpm coverage
• pnpm exec playwright test tests/e2e/chat.spec.ts --grep "unread bar jumps|read history returns|automatic read receipts|message history pages"
• git diff --check