Fix deleted vs not-found comment state

[GaboHBeaumont]

Summary

• use a direct subscribed resource lookup for the opened comment in CommentDiscussions
• distinguish exact focused comment states: render comment, deleted preview, deleted, loading, or not-found
• add unit tests for the focused comment state resolver

Testing

• not run: frontend dependencies are not installed in this workspace (node_modules missing)
• attempted corepack pnpm --filter @shm/ui typecheck
• attempted corepack pnpm --filter @shm/ui test

[ericvicenti]

@GaboHBeaumont if you have tested this, and once the tests are passing, feel free to merge!

btw, this getFocusedCommentViewState function does not look very helpful and I would prefer if that code was just inlined at the place where it is being used. a helper function is only helpful if the logic is being used in more than one place

I have a feeling it was done this way because of the agent's eagerness to write another super-obvious test 😂

[GaboHBeaumont]

Great! thanks!

[GaboHBeaumont]

@ericvicenti done — I inlined that state logic in and removed the extra helper + test.

[GaboHBeaumont]

@ericvicenti done — I inlined that state logic in comments.tsx and removed the extra helper + test.