fix(useGetMessages): long poll messages without browser freeze

[Antreesy]

☑️ Resolves

• Ref chatBlocks follow-ups #15668
  • fix initial scroll position (any chat), extra chat requests from being cose to threshold, 'No messages' initially appearing (sidebar integration)
  • check if getNewMessages (from scroll threshold) tries to make parallel request for same messages as polling and cancel it
  • add function to set context to last message from last read message (without route change)
• Fix Lots of message, can't load #15235
  • add conversation last message as separate context to make polling from it and do not kill the browser with a lot of unread messages fetch

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• ⛑️ Tests are included or not possible