fix(navigation): hold a neutral surface during the tab-switch activation gap

[mremond]

Summary

Switching content tabs (Messages / Rooms / Archive) via the rail icons briefly flashed the empty-state hero before the conversation or room rendered.

navigateToView() synchronously flips sidebarView and clears the opposite store, then fires the hydrating activateConversation / activateRoom action. Those await an IndexedDB cache read before setting the active id — by design, so the view never renders with an empty message list and the unread-divider is computed against loaded messages. That await is a real macrotask, so React commits and paints the in-between frame where both activeConversationId and activeRoomJid are null while sidebarView is already a content view, and the render cascade falls through to the EmptyState hero. The lazy tabs (Settings / Admin / Search) never showed this because they sit behind a Suspense skeleton; the content tabs had nothing covering the gap.

This adds a token-correct activationPending flag to chatStore and roomStore — set synchronously before the cache-read await and cleared atomically when the active id lands (the existing monotonic activation token means a superseded activation leaves the flag for the winner to clear). ChatLayout renders the existing ViewLoadingFallback skeleton instead of the EmptyState hero while it is true, so content-tab switches now match the lazy tabs and the hero no longer pops in and out.

activationPending is also a useful signal for any client built on the SDK, which hits the same flash.