Dashboard: Fix Activity widget text overflow. Fixes #64827

[meravi]

Prevents long text in the Dashboard Activity widget from overflowing its container.

This adjusts the widget CSS so long, unbroken strings wrap correctly in activity items and links.

Testing:

1. Open the Dashboard Activity widget.
2. Check an item with long unbroken text.
3. Confirm the text wraps instead of overflowing.

Trac ticket: https://core.trac.wordpress.org/ticket/64827

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Props ravikhadka.

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