feat(phoenix-ng): add NotificationService with 4 severity levels and configurable auto-dismiss

[deveshbervar]

Summary

Adds NotificationService as part of Work Area 1 (WA1).

WA1 arc:

• Step 1 — IO error handling with try/catch (fix(io-options-dialog): surface JSON parse errors to user via info logger (#842) #879, merged)
• Step 2 — NotificationService (this PR)
• Step 3 — UI component that subscribes and shows toasts(next part)

What this adds

notification.service.ts — Injectable Angular service:

notificationService.success('File loaded successfully.');
notificationService.info('Loading event data...');
notificationService.warning('File may be malformed.');
notificationService.error('Could not parse event file.');

Follows the same ActiveVariable pattern as ErrorMessageService.
Components subscribe via subscribeToNotifications().

Default auto-dismiss durations:

• success — 5000ms
• info — 5000ms
• warning — 8000ms
• error — 0ms (manual dismiss required)

Custom duration supported via optional second parameter.

notification.service.component.test.ts — 6 tests:

• All 4 severity levels emit correct message and severity
• Default durations per severity level
• Custom duration override
• Multiple subscribers supported

Why

PR #879 added try/catch to route IO errors through InfoLogger.
This PR adds the service layer that allows any Phoenix component
to emit user-visible notifications programmatically.

The next step (separate PR) will add the UI component that
subscribes to this service and renders toasts via MatSnackBar.

Scope

• 2 new files in phoenix-ui-components/lib/services/
• 1 line added to index.ts for export
• No changes to existing files beyond the export
• 6 tests passing locally