feat: undo button for trip planner

Implements a full undo history system for the Plan screen.

New hook: usePlannerHistory (client/src/hooks/usePlannerHistory.ts)
- Maintains a LIFO stack (up to 30 entries) of reversible actions
- Exposes pushUndo(label, fn), undo(), canUndo, lastActionLabel

Tracked actions:
- Assign place to day (undo: remove the assignment)
- Remove place from day (undo: re-assign at original position)
- Reorder places within a day (undo: restore previous order)
- Move place to a different day (undo: move back)
- Optimize route (undo: restore original order)
- Lock / unlock place (undo: toggle back)
- Delete place (undo: recreate place + restore all day assignments)
- Add place (undo: delete it)
- Import from GPX (undo: delete all imported places)
- Import from Google Maps list (undo: delete all imported places)

UI: Undo button (Undo2 icon) in DayPlanSidebar header. PDF, ICS and
Undo buttons all use custom instant hover tooltips instead of native
title attributes.

A toast notification confirms each undo action.

Translations: undo.* keys added to all 12 language files.

client/src/components/Planner/PlacesSidebar.tsx

@@ -29,11 +29,12 @@ interface PlacesSidebarProps {
   days: Day[]
   isMobile: boolean
   onCategoryFilterChange?: (categoryId: string) => void
+  pushUndo?: (label: string, undoFn: () => Promise<void> | void) => void
 }
 
 const PlacesSidebar = React.memo(function PlacesSidebar({
   tripId, places, categories, assignments, selectedDayId, selectedPlaceId,
-  onPlaceClick, onAddPlace, onAssignToDay, onEditPlace, onDeletePlace, days, isMobile, onCategoryFilterChange,
+  onPlaceClick, onAddPlace, onAssignToDay, onEditPlace, onDeletePlace, days, isMobile, onCategoryFilterChange, pushUndo,
 }: PlacesSidebarProps) {
   const { t } = useTranslation()
   const toast = useToast()
@@ -52,6 +53,15 @@ const PlacesSidebar = React.memo(function PlacesSidebar({
       const result = await placesApi.importGpx(tripId, file)
       await loadTrip(tripId)
       toast.success(t('places.gpxImported', { count: result.count }))
+      if (result.places?.length > 0) {
+        const importedIds: number[] = result.places.map((p: { id: number }) => p.id)
+        pushUndo?.(t('undo.importGpx'), async () => {
+          for (const id of importedIds) {
+            try { await placesApi.delete(tripId, id) } catch {}
+          }
+          await loadTrip(tripId)
+        })
+      }
     } catch (err: any) {
       toast.error(err?.response?.data?.error || t('places.gpxError'))
     }
@@ -70,6 +80,15 @@ const PlacesSidebar = React.memo(function PlacesSidebar({
       toast.success(t('places.googleListImported', { count: result.count, list: result.listName }))
       setGoogleListOpen(false)
       setGoogleListUrl('')
+      if (result.places?.length > 0) {
+        const importedIds: number[] = result.places.map((p: { id: number }) => p.id)
+        pushUndo?.(t('undo.importGoogleList'), async () => {
+          for (const id of importedIds) {
+            try { await placesApi.delete(tripId, id) } catch {}
+          }
+          await loadTrip(tripId)
+        })
+      }
     } catch (err: any) {
       toast.error(err?.response?.data?.error || t('places.googleListError'))
     } finally {