Pick sidebar fallback thread after delete

[juliusmarminge]

• Choose the next visible thread by sidebar sort order
• Skip other threads deleted in the same action

What Changed

Why

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Low risk: changes only client-side navigation after thread deletion, with added unit tests; main risk is incorrect fallback selection in edge cases (e.g., bulk delete or cross-project threads).

Overview
When deleting the currently viewed thread, the sidebar now navigates to a deterministic fallback thread: the top remaining thread in the same project according to the configured sidebar thread sort order, while skipping any other threads being deleted in the same bulk action.

This introduces getFallbackThreadIdAfterDelete in Sidebar.logic.ts, uses it from Sidebar.tsx during thread deletion (including adding sidebarThreadSortOrder to the callback deps), and adds targeted tests covering sort-order selection and bulk-delete skipping.

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Note

Pick fallback thread from same project when deleting the active sidebar thread

• Adds getFallbackThreadIdAfterDelete in Sidebar.logic.ts to find the top remaining thread in the deleted thread's project, sorted by sidebarThreadSortOrder, skipping any other threads being deleted in the same action.
• Updates Sidebar.tsx to use this function instead of the previous logic, which could navigate to any first available thread across all projects.
• Behavioral Change: after deleting the active thread, navigation now targets the same project's next thread rather than the globally first available thread.

^{Macroscope summarized b671f72.}

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