fix(web): add activities to default apps list

[michaelstingl]

Description

Adds activities to the default web apps list in defaultconfig.go.

Related Issue

• Fixes web-app-activities bundled but missing from default apps list #2089

Motivation and Context

The web-app-activities is bundled in OpenCloud Web but not loaded by default because activities is missing from the Apps array. The backend activitylog service runs by default, but users cannot access the global activities view.

How Has This Been Tested?

• test environment: opencloud-compose with OpenCloud 4.1.0, Web 4.3.0
• test case 1: Verified activities appears in /config.json apps array after config override
• test case 2: Activities app visible in app menu after restart

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation added