[AlexAndBear]

Description

The notifications are now only removed after a successful delete; a failed request shows an error instead of silently emptying the list.

Related Issue

• Fixes A failed "Mark all as read" silently empties the notification list #3883

How Has This Been Tested?

• test environment: unit tests (Vitest), browser tests (Playwright) against a local OpenCloud
• test case 1: unit: a failed delete keeps the notifications and shows an error
• test case 2: browser: failed request (500 and network error) keeps list and badge and shows the error; success empties the list; new notifications still arrive

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)