Conversations in the sidebar were being sorted by the timestamp of the
last message (including assistant responses). This changes the sorting
to use only user messages, matching the behavior of ChatGPT and Claude.
Fixes#12958
* app: add code for macOS and Windows apps under 'app'
* app: add readme
* app: windows and linux only for now
* ci: fix ui CI validation
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Co-authored-by: jmorganca <jmorganca@gmail.com>