Fix notification-triggered search reloads and sidebar flicker

[gbirman]

Summary

• Narrowed soup query invalidation on notifications: Channel and document notification handlers now use refetchSoupEntity + invalidateSoupEntity (per-entity) instead of invalidating all soup queries via soupKeys._def. This prevents incoming notifications from retriggering unrelated search requests.
• Stabilized unread channel ordering in sidebar: The unread channels list now maintains a stable order — channels only reposition when they enter/leave the unread set, not when notification counts update during refetches.
• Filtered out currently-viewed channels from unread list: Channels open in any split are excluded from the unread widget, preventing the brief flash of unread 1 when receiving a notification in a channel you are already viewing.

[github-actions]

Preview: https://gab-fix-notification-search-re-lg3nrs.preview.macro.com/app (f223d5c)