Adds sticky `lastSelectedSpaceId` to useSpaceStore. Updated on every
setCurrentSpace(non-null) and on loadSpaceDetail success; NOT cleared by
setCurrentSpace(null), so the @me URL effect in AppLayout no longer wipes
the memory. Cleared only when the remembered space is actually removed
(deleteSpace / leaveSpace / removeSpace / removeInstanceSpaces / reset).
Ephemeral — URL drives initial state on reload.
MobileBottomNav.handleTab('spaces') and MobileSpacesScreen's initial
useState now resolve via `currentSpaceId ?? lastSelectedSpaceId`. The
prior Object.keys(lastChannelPerSpace)[0] fallback was wrong twice over:
first-inserted ordering locked the tab to whichever space the user
opened first in their session, and any approach reading currentSpaceId
alone breaks after a /channels/@me detour.
Also fixes a pre-existing render loop in MobileSpacesScreen's
currentSpaceId sync effect — `selectedSpaceId` removed from deps,
functional setState makes the update idempotent.
Spec: docs/systems/mobile-ui.md updates Tab Tap Behavior, MobileScreenHeader,
and LocalStorage Persistence sections.
Jannis Braun
d9c9f18b20