Sort pinned sessions to the top of the conversations list

[insign]

Pinned sessions now correctly appear at the top of the session list instead of remaining mixed with unpinned sessions, restoring the expected UX behavior of the pinning feature. This allows users to quickly access important or frequently used conversations without having to scroll down.

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PR created automatically by Jules for task 491606797168416803 started by @insign

[google-labs-jules]

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