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/services/photoService.ts

@@ -0,0 +1,128 @@
+import { mapsApi } from '../api/client'
+
+// Shared photo cache — used by PlaceAvatar (sidebar) and MapView (map markers)
+interface PhotoEntry {
+  photoUrl: string | null
+  thumbDataUrl: string | null
+}
+
+const cache = new Map<string, PhotoEntry>()
+const inFlight = new Set<string>()
+const listeners = new Map<string, Set<(entry: PhotoEntry) => void>>()
+// Separate thumb listeners — called when thumbDataUrl becomes available after initial load
+const thumbListeners = new Map<string, Set<(thumb: string) => void>>()
+
+function notify(key: string, entry: PhotoEntry) {
+  listeners.get(key)?.forEach(fn => fn(entry))
+  listeners.delete(key)
+}
+
+function notifyThumb(key: string, thumb: string) {
+  thumbListeners.get(key)?.forEach(fn => fn(thumb))
+  thumbListeners.delete(key)
+}
+
+export function onPhotoLoaded(key: string, fn: (entry: PhotoEntry) => void): () => void {
+  if (!listeners.has(key)) listeners.set(key, new Set())
+  listeners.get(key)!.add(fn)
+  return () => { listeners.get(key)?.delete(fn) }
+}
+
+// Subscribe to thumb availability — called when base64 thumb is ready (may be after photoUrl)
+export function onThumbReady(key: string, fn: (thumb: string) => void): () => void {
+  if (!thumbListeners.has(key)) thumbListeners.set(key, new Set())
+  thumbListeners.get(key)!.add(fn)
+  return () => { thumbListeners.get(key)?.delete(fn) }
+}
+
+export function getCached(key: string): PhotoEntry | undefined {
+  return cache.get(key)
+}
+
+export function isLoading(key: string): boolean {
+  return inFlight.has(key)
+}
+
+// Convert image URL to base64 via canvas (CORS required — Wikimedia supports it)
+function urlToBase64(url: string, size: number = 48): Promise<string | null> {
+  return new Promise(resolve => {
+    const img = new Image()
+    img.crossOrigin = 'anonymous'
+    img.onload = () => {
+      try {
+        const canvas = document.createElement('canvas')
+        canvas.width = size
+        canvas.height = size
+        const ctx = canvas.getContext('2d')!
+        const s = Math.min(img.naturalWidth, img.naturalHeight)
+        const sx = (img.naturalWidth - s) / 2
+        const sy = (img.naturalHeight - s) / 2
+        ctx.beginPath()
+        ctx.arc(size / 2, size / 2, size / 2, 0, Math.PI * 2)
+        ctx.clip()
+        ctx.drawImage(img, sx, sy, s, s, 0, 0, size, size)
+        resolve(canvas.toDataURL('image/webp', 0.6))
+      } catch { resolve(null) }
+    }
+    img.onerror = () => resolve(null)
+    img.src = url
+  })
+}
+
+export function fetchPhoto(
+  cacheKey: string,
+  photoId: string,
+  lat?: number,
+  lng?: number,
+  name?: string,
+  callback?: (entry: PhotoEntry) => void
+) {
+  const cached = cache.get(cacheKey)
+  if (cached) { callback?.(cached); return }
+
+  if (inFlight.has(cacheKey)) {
+    if (callback) onPhotoLoaded(cacheKey, callback)
+    return
+  }
+
+  inFlight.add(cacheKey)
+  mapsApi.placePhoto(photoId, lat, lng, name)
+    .then(async (data: { photoUrl?: string }) => {
+      const photoUrl = data.photoUrl || null
+      if (!photoUrl) {
+        const entry: PhotoEntry = { photoUrl: null, thumbDataUrl: null }
+        cache.set(cacheKey, entry)
+        callback?.(entry)
+        notify(cacheKey, entry)
+        return
+      }
+
+      // Store URL first — sidebar can show immediately
+      const entry: PhotoEntry = { photoUrl, thumbDataUrl: null }
+      cache.set(cacheKey, entry)
+      callback?.(entry)
+      notify(cacheKey, entry)
+
+      // Generate base64 thumb in background
+      const thumb = await urlToBase64(photoUrl)
+      if (thumb) {
+        entry.thumbDataUrl = thumb
+        notifyThumb(cacheKey, thumb)
+      }
+    })
+    .catch(() => {
+      const entry: PhotoEntry = { photoUrl: null, thumbDataUrl: null }
+      cache.set(cacheKey, entry)
+      callback?.(entry)
+      notify(cacheKey, entry)
+    })
+    .finally(() => { inFlight.delete(cacheKey) })
+}
+
+export function getAllThumbs(): Record<string, string> {
+  const r: Record<string, string> = {}
+  for (const [k, v] of cache.entries()) {
+    if (v.thumbDataUrl) r[k] = v.thumbDataUrl
+  }
+  return r
+}