🔗 fix: Add branch-specific shared links (targetMessageId)

[berry-13]

Summary

Add support for sharing a specific branch of a conversation by allowing users to generate shared links that reference a particular message (targetMessageId). Backend: added targetMessageId to the ISharedLink schema and model, introduced a compound index, updated createSharedLink and the API route to handle targetMessageId and filter messages up to the target. Frontend: updated useCreateSharedLinkMutation and dataService.createSharedLink to accept targetMessageId, and enhanced ShareButton / SharedLinkButton components to use the latest message's ID as targetMessageId

Closes: #10007

Change Type

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

Testing

started a new convo, shared it, loaded the shared convo, created a message branch, and refreshed the loaded convo (verified as working)

Test Configuration:

• OS: WSL2
• npm Version: 11.5.2

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.