feat(ui): promote archive to an envelope primary action

[mrx8086]

Summary

Favorite, read and important are available as one-click primary action
buttons at the top of the envelope actions menu, but archiving — a core
step of inbox-zero style workflows — sits further down the entry list.

This PR promotes the existing archive entry to a fourth primary action
button:

• Reuses the existing onArchive() handler, showArchiveButton /
  disableArchiveButton computed properties and the hasArchiveAcl
  check — no behavioral changes.
• The button only appears when an archive mailbox is configured for the
  account and the ACLs allow it, exactly like the previous menu entry.
• The entry is moved (not duplicated): the old list entry is removed so
  the same action does not appear twice in one open menu.
• No new translatable strings — the existing "Archive thread" /
  "Archive message" strings are reused.

If the design team prefers to keep the primary row at three buttons, I
am happy to adjust (e.g. make the fourth button conditional or move it
behind a setting) — feedback welcome.

Screenshots

Screenshots of the envelope actions menu before/after will follow.

Checklist

• Code is properly formatted (npx eslint src/components/Envelope.vue — no new warnings)
• Frontend builds (npm run build)
• Sign-off message and AI-assisted trailer are added

[welcome]

Thanks for opening your first pull request in this repository! ✌️

[ChristophWurst]

Thank you for the contribution

@nimishavijay fine by you?

@mrx8086 please describe the changes in your words next time and don't use AI for the PR description: https://github.com/nextcloud/.github/blob/master/CONTRIBUTING.md#ai-assisted-contributions
The commit trailer format for AI is also slightly outdated. https://github.com/nextcloud/.github/blob/master/AI_POLICY.md#disclosure is the format we use now.

[mrx8086]

Thank you, and sorry about that — I'll write the descriptions and replies personally going forward. The commit trailer is updated to the current Assisted-by: format.

[nimishavijay]

Fine by me, but keep in mind #13310 which adds reply also. Reply should be the first option and then archive.

[mrx8086]

Thanks for the feedback, I agree. I'll wait until the reply PR (#13310) is merged, then I'll rebase and put the archive button after reply. Thanks!