feat: Declare the Agenda digest mail notifications category - EXO-89484 - eXIP7.3.0.22

[azayati]

Part of eXIP 7.3.0.22 — Digest Mail Notifications. Task EXO-89484 (US03).

Contributes the Agenda category to the digest settings drawer. Depends on Meeds-io/commons#772, which brings the Kernel DigestCategoryRegistry this configuration targets — merge that one first.

Pure configuration, no Java:

• The category is declared as a component-plugin on the registry, next to the notification plugins it already declares, and covers the 7 agenda plugins (event added/modified/canceled, reminder, reply, date poll, vote).
• Its label is added to the resource bundle those plugins already use, where social's DigestCategoryLabelResolver reads it — translated server side, so the settings page has nothing to load.

The categories are declared in the Kernel rather than as Spring beans on purpose: each webapp has its own Spring context, and only the Kernel container is shared by all of them. This addon's category therefore reaches the digest whatever the addon is made of.

AI contribution

Classified N1 — human-driven, no auto-merge, author ≠ approver.

[azayati]

Second commit added — the timezone delegation from the tech spec (§4 Timezone).

The timezone synchronization is now owned by the platform: social publishes eXo.env.portal.userTimezone on every page, refreshes it through POST /social/rest/timezone backed by UserTimeZoneService.saveUserTimeZone(username, zoneId), and broadcasts an event on every change (a ProfileListenerPlugin never fires for this attribute — rejected alternative 7 of the spec).

Per the spec, agenda delegates and deprecates its own copies:

• AgendaUserSettingsService.updateUserTimeZone → deprecated, delegates to the social service so the timezone events are never skipped
• agenda's TimeZoneRest → deprecated, stays for existing callers, delegates the same way
• the head script publishing eXo.env.portal.userTimezone and the page sync script are removed — they duplicate what social now does on every page (§5 cleanup: 'agenda's timezone-sync copies once they delegate to the platform sync')

This PR now additionally requires the social PR Meeds-io/social#6027 to be merged first (it brings UserTimeZoneService and the platform-side sync). All 8 AgendaUserSettingsServiceTest tests green.

[azayati]

Strategy change on the timezone part — full removal instead of delegate-and-deprecate.

Decision: the spec §5 cleanup is done progressively within each task (only a large cleanup chunk would wait), so agenda's timezone-sync code is removed entirely in this PR rather than deprecated:

• UIPortalAgendaHead.gtmpl + its webui registration, and the agenda-base-extension sync bundle — duplicated by what social now does on every page (feat: Digest mail notifications settings drawer - EXO-89484 - eXIP7.3.0.22 Meeds-io/social#6027)
• TimeZoneRest and AgendaUserSettingsService.updateUserTimeZone (API + impl + container test) — their only caller was that page script; verified by grep across the workspace: nothing calls the endpoint or the method any more
• the now-unused OrganizationService dependency of AgendaUserSettingsServiceImpl goes with them

The branch was rewritten so the PR tells the final story (3 commits: category, removal, spec plugin list). AgendaUserSettingsServiceTest: 7 tests green. Still depends on Meeds-io/social#6027 merging first (social must publish the sync before agenda's copy disappears from a deployed platform).