Brings mobile to skeleton parity with desktop. Two mobile-specific renders, plus a fix for the empty-state mascot flashing on every space switch.
- MobileSpacesScreen: shimmer skeleton for the channel list while `useSpaceStore.loadingSpaceId === selectedSpaceId` (uncategorized rows + category header + categorized rows). Mirrors desktop ChannelSidebar's `showChannelSkeleton`. Gated through `useDelayedLoading` so cached/fast loads don't flash the placeholder.
- MobileMembersScreen: shimmer skeleton for the member list (two role-section headers + circular avatars + name bars). Mirrors desktop MemberSidebar's `showMemberSkeleton`.
- Empty-state "No channels yet." mascot was firing on EVERY space switch for ~50–200 ms — `state.channels` only ever holds the most-recently loaded space's channels, so `spaceChannels` filters to `[]` immediately on selection change while `loadSpaceDetail` is still in flight. New `loadedSpaceIds: Set<string>` on useSpaceStore, populated only on `loadSpaceDetail` success and pruned on space removal events. The mascot is now gated on `loadedSpaceIds.has(selectedSpaceId)` so it only fires when the space is *settled* with zero channels.
Reuses the existing `.skeleton` / `.skeleton-bar` / `.skeleton-circle` CSS primitives + `useDelayedLoading` hook (no new shared `<Skeleton />` primitive — sites need row-specific geometry, parameterizing would either over-abstract or duplicate the inline approach).
Specs: docs/systems/mobile-ui.md (Loading Skeletons inventory + per-screen skeleton bullets + settle-gate rationale); docs/systems/spaces.md (Client Load State subsection covering loadingSpaceId vs loadedSpaceIds).
Jannis Braun
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