perf: major trip planner performance overhaul (#218)

Store & re-render optimization:
- TripPlannerPage uses selective Zustand selectors instead of full store
- placesSlice only updates affected days on place update/delete
- Route calculation only reacts to selected day's assignments
- DayPlanSidebar uses stable action refs instead of full store

Map marker performance:
- Shared photoService for PlaceAvatar and MapView (single cache, no duplicate requests)
- Client-side base64 thumbnail generation via canvas (CORS-safe for Wikimedia)
- Map markers use base64 data URL <img> tags for smooth zoom (no external image decode)
- Sidebar uses same base64 thumbnails with IntersectionObserver for visible-first loading
- Icon cache prevents duplicate L.divIcon creation
- MarkerClusterGroup with animate:false and optimized chunk settings
- Photo fetch deduplication and batched state updates

Server optimizations:
- Wikimedia image size reduced to 400px (from 600px)
- Photo cache: 5min TTL for errors (was 12h), prevents stale 404 caching
- Removed unused image-proxy endpoint

UX improvements:
- Splash screen with plane animation during initial photo preload
- Markdown rendering in DayPlanSidebar place descriptions
- Missing i18n keys added, all 12 languages synced to 1376 keys

client/src/hooks/useRouteCalculation.ts

@@ -19,7 +19,8 @@ export function useRouteCalculation(tripStore: TripStoreState, selectedDayId: nu
   const updateRouteForDay = useCallback(async (dayId: number | null) => {
     if (routeAbortRef.current) routeAbortRef.current.abort()
     if (!dayId) { setRoute(null); setRouteSegments([]); return }
-    const da = (tripStore.assignments[String(dayId)] || []).slice().sort((a, b) => a.order_index - b.order_index)
+    const currentAssignments = tripStore.assignments || {}
+    const da = (currentAssignments[String(dayId)] || []).slice().sort((a, b) => a.order_index - b.order_index)
     const waypoints = da.map((a) => a.place).filter((p) => p?.lat && p?.lng)
     if (waypoints.length < 2) { setRoute(null); setRouteSegments([]); return }
     setRoute(waypoints.map((p) => [p.lat!, p.lng!]))
@@ -33,12 +34,14 @@ export function useRouteCalculation(tripStore: TripStoreState, selectedDayId: nu
       if (err instanceof Error && err.name !== 'AbortError') setRouteSegments([])
       else if (!(err instanceof Error)) setRouteSegments([])
     }
-  }, [tripStore, routeCalcEnabled])
+  }, [routeCalcEnabled])
 
+  // Only recalculate when assignments for the SELECTED day change
+  const selectedDayAssignments = selectedDayId ? tripStore.assignments?.[String(selectedDayId)] : null
   useEffect(() => {
     if (!selectedDayId) { setRoute(null); setRouteSegments([]); return }
     updateRouteForDay(selectedDayId)
-  }, [selectedDayId, tripStore.assignments])
+  }, [selectedDayId, selectedDayAssignments])
 
   return { route, routeSegments, routeInfo, setRoute, setRouteInfo, updateRouteForDay }
 }