fix(vue3): restore global t/n helpers on app instances

[pringelmann]

Resolves: #1547

Summary

GuestSettings, GuestForm, LanguageSelect and GroupSelect call t() directly in their templates but neither import t nor expose it via methods after the Vue 3 migration removed the global Nextcloud.js mixin. The admin Guests page renders blank and the file Sharing "Invite guest" dropdown silently fails with TypeError: e.t is not a function.

Import t from @nextcloud/l10n in each affected SFC and register it on the component's methods so the template proxy can resolve it. Same per-component pattern already used in GuestList.vue, GuestDetails.vue and TransferGuestDialog.vue.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[pringelmann]

@skjnldsv this is still in draft, but please take a look so long if you have a minute

[skjnldsv]

Hey @pringelmann !
The goal is to remove than antipattern. They should be registered on each component directly.

[pringelmann]

@skjnldsv sure thing, but the latest guests release is completely broken as a result of this regression. So can I suggest we merge this and I open a follow-up PR to register in each component?

[skjnldsv]

I started a copilot agent to work on this, I'll ping you in the PR

[skjnldsv]

#1552 @pringelmann

[pringelmann]

@skjnldsv I've updated the PR with the correct fix. Would prefer to get this out as a clean patch asap if that's OK? (as opposed to it being rolled into a bigger PR)

[skjnldsv]

Yes and no, there are other issues needed to be fixed.

[pringelmann]

@skjnldsv fair, will review the copilot PR so we can release everything together