fix: minor UX enhancements

[Antreesy]

☑️ Resolves

• LeftSidebar: get rid of forced scrollbars
• NewMessageUploadEditor: adjust design, add hover effect

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
Screenshot before

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
  Signed-off-by: Antreesy antreesy.web@gmail.com

[DorraJaouad]

Tested, looks good

[Antreesy]

/backport to stable30

[ShGKme]

• LeftSidebar: get rid of forced scrollbars

It was there intentionally (in the list). Getting rid of it returns layout shift when scrollbar appears/disappears and makes conversation item not inline other items in the navigation.

[DorraJaouad]

It was there intentionally (in the list). Getting rid of it returns layout shift when scrollbar appears/disappears and makes conversation item not inline other items in the navigation.

Maybe it's acceptable like that because invitation counter and navigation items do not belong to the scrollable list and they stay sticky ?

[Antreesy]

I have a wip commit to calculate user agent width of scrollbar and increase space on the right by that value to align - but does it worth it?