fix(MessagesList): update date separator overnight

[DorraJaouad]

☑️ Resolves

• Fix: when keeping the conversation open overnight, relative dates do not get updated.

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
messages list showing two "today", one of them is supposed to be "yesterday"	only one correct "today"

🏁 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

[nickvergessen]

/backport to stable30

[nickvergessen]

/backport to stable29

[Antreesy]

After private discussion:

• move logic to onWindowFocus()
• use Vue.set() to ensure update is reactive
• use :key="dateSeparatorLabels[dateTimestamp]" in template if won't update without it

Tried that way, requires 1 minute for browser to grab a changed system time, then click on window trigger re-render

[Antreesy]

Working after locking/unlocking the screen, as demanded.
Check linter and merge =)

[ShGKme]

• use :key="dateSeparatorLabels[dateTimestamp]" in template if won't update without it

Why?