Optimize conversation unread message selectors

[onuraycicek]

Summary

• Avoid constructing Set instances and wrapper objects inside useAppSelector for unread conversation counts.
• Memoize unread-count calculations from stable Redux slices instead, so unrelated store updates do not force these consumers to re-render.
• Reuse a stable empty messages array for missing conversations to avoid creating a fresh [] selector result.

Why

This targets the UI performance bounty in #727. The previous selectors returned new references on every Redux update even when conversations and seenMessages were unchanged, which makes React-Redux treat the selected value as changed and re-render subscribers unnecessarily.

Testing

• corepack yarn tsc
• corepack yarn eslint src/store/hooks.ts src/renderer/components/pages/help/ConversationsBox.tsx --quiet

[Einliterflasche]

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