Add optional top bar date/time with personal visibility toggle

[Zhey-on]

Summary

Fixes #38726.

This adds an optional date and time label in the top bar next to the account menu.
This is intentionally not a dashboard widget.

Users can enable or disable it from Personal settings > Appearance and accessibility settings, under a new Visibility options switch.

What changed

• Added a reusable HeaderDateTime component for top bar rendering.
• Integrated it in the account menu area, not in Unified Search.
• Added a new personal setting: Show date and time in top bar.
• Persisted the setting as theming/show_header_datetime with yes/no values.
• Exposed the setting via initial state on authenticated pages.
• Allowed and validated the new preference key in theming preference validation.
• Kept runtime behavior live by emitting and listening to an event bus update.
• Updated built frontend assets.

Testing

• npm run build -- --filter=core
• php -l apps/theming/lib/Listener/BeforeTemplateRenderedListener.php
• php -l apps/theming/lib/Settings/Personal.php
• php -l apps/theming/lib/Listener/BeforePreferenceListener.php

[susnux]

Should rather just use NcDateTime from nextcloud-vue

[susnux]

new files should - if possible - use script setup syntax to be future proof

[susnux]

I do not think this belongs to the account menu

[susnux]

how are those changes related?

[susnux]

how is this related?

[susnux]

Thank you for your contribution!
But please use the pull request template to make review easier and to keep all required todo fields.

Could you please add screen shots so we can get a review of the @nextcloud/designers to decide if this is something we want to have in the product :)

[kra-mo]

I don't think we want this from a design POV. Every operating system already has a way to show the date and time in their own bar. The dashboard? Sure, maybe. It has the advantage of potentially actually being front-and-center unlike the time from the OS. But this seems borderline useless.

[susnux]

The dashboard? Sure, maybe. It has the advantage of potentially actually being front-and-center unlike the time from the

I agree as in the dashboard we could then also show timezone offset (e.g. if you are on a public device where local time is 10 am but in Nextcloud you configured a timezone so its 11am.).