NMS-20100: PrimeVue Notifications page with a General configuration tab (base)

[joseanesONMS]

Rewrites the Notifications page as PrimeVue backed by REST, split from the original single PR for ease of review. This base PR carries the page itself and a Configure Notifications dialog with only the General tab; the Event Notifications, Destination Paths and Path Outages tabs follow in dependent PRs.

• Notifications page: notice queries (per-user, outstanding, acknowledged), the outstanding-notices list with acknowledge, CSV export and printing, and collapsible explanations.
• Configure Notifications dialog (gear icon) with the General tab: the system-wide notification on/off switch.
• Adds admin-only /rest/notification-config endpoints for the notifd status plus the read side of destination paths (shared by the dependent tab PRs), delegating to the same file-backed factories the legacy wizards use — the XML files stay the system of record.
• Browser delivery of notices for /ui pages.
• The admin menu entry points at the new page.

Dependent PRs: Event Notifications tab, Destination Paths tab, Path Outages tab (each contains this PR's commit until it merges).

[cgorantla]

@joseanesONMS This is not running any tests as it is coming from fork, can we directly create branch on this repo and open PR ?

[joseanesONMS]

@cgorantla Happy to — but I don't currently have write access to OpenNMS/opennms (push returns 403 for joseanesONMS), so I can't create in-repo branches myself. If you can grant me write access I'll recreate both this PR and #8692 from in-repo branches right away; alternatively a maintainer with access could push the branch (maintainer edits are allowed on this PR). In the meantime I'm adding unit/component tests to the PR.

[joseanesONMS]

Tests added in 0640aad: NotificationConfigRestServiceIT (full REST surface: notifd status, event notification / destination path lifecycles, path outage apply/clear/preview, admin guard — 8 tests, follows the ScheduledOutagesRestServiceIT pattern with seeded config files and a mocked FilterDao) plus Vitest suites for the two new Pinia stores (25 tests). All green locally: mvn -pl opennms-webapp-rest verify -Dit.test=NotificationConfigRestServiceIT -DskipITs=false and cd ui && vitest run tests/stores. Still happy to move this to an in-repo branch as soon as I have write access.

[joseanesONMS]

Rebased onto current develop: the UI seam layer work (NMS-20029/NMS-20081) moved ConfirmationDialog and OnmsIconButton into @opennms/onms-ui, which this PR's components imported from their old location. Adapted in the final commit (same APIs, new imports); everything re-verified on the rebased branch — 15 integration tests, 26 store tests, full UI typecheck/build.

[joseanesONMS]

Replaced by #8709 (in-repo branch on the jira/NMS-20100 epic feature branch, so CI runs).