fix: remove dashboard activity view

[walker-tx]

Depends on #5968. Review and land that PR first.

Summary

• removes the embedded Activity section from Dashboard Platform Admin Overview
• deletes the superseded Dashboard Activity component and its dedicated tests
• keeps organization details, support-session controls, and the development-only platform-admin control covered on Overview

Motivation

Dashboard Activity depended on an Admin-authenticated endpoint that Dashboard sessions cannot call. With the richer general organization audit feed consolidated in the standalone Admin UI by the parent PR, retaining this broken duplicate would be misleading and unnecessary.

[changeset-bot]

⚠️ No Changeset found

Latest commit: e59bba6

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[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}

[walker-tx]

Demo

What it shows:

1. Platform Admin Overview still renders organization details.
2. Support-session and development-only controls remain available.
3. The superseded Dashboard Activity section is no longer present.