fix(journey): keep page mounted during in-place journey refetch

loadJourney previously set loading=true unconditionally, causing the
JourneyDetailPage guard (if loading || !current) to unmount the entire
page tree on every background refetch — entry saves, settings saves,
trip link/unlink, contributor invite, delete, and WS realtime events
all triggered the full-page spinner flash.

Now loading is only toggled on cold loads (current?.id !== id).
Warm refreshes replace current silently so the hero, sidebar, map,
and timeline stay mounted throughout. Closes #673.

client/src/store/journeyStore.ts

@@ -124,7 +124,8 @@ export const useJourneyStore = create<JourneyState>((set, get) => ({
   },
 
   loadJourney: async (id) => {
-    set({ loading: true, notFound: false })
+    const cold = get().current?.id !== id
+    if (cold) set({ loading: true, notFound: false })
     try {
       const data = await journeyApi.get(id)
       set({ current: data })
@@ -134,7 +135,7 @@ export const useJourneyStore = create<JourneyState>((set, get) => ({
       }
       throw err
     } finally {
-      set({ loading: false })
+      if (cold) set({ loading: false })
     }
   },