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/i18n/translations/nl.ts

@@ -1484,6 +1484,19 @@ const nl: Record<string, string> = {
   'perm.actionHint.packing_edit': 'Wie kan pakitems en tassen beheren',
   'perm.actionHint.collab_edit': 'Wie kan notities, polls aanmaken en berichten versturen',
   'perm.actionHint.share_manage': 'Wie kan openbare deellinks aanmaken of verwijderen',
+  // Undo
+  'undo.button': 'Undo',
+  'undo.tooltip': 'Undo: {action}',
+  'undo.assignPlace': 'Place assigned to day',
+  'undo.removeAssignment': 'Place removed from day',
+  'undo.reorder': 'Places reordered',
+  'undo.optimize': 'Route optimized',
+  'undo.deletePlace': 'Place deleted',
+  'undo.moveDay': 'Place moved to another day',
+  'undo.lock': 'Place lock toggled',
+  'undo.importGpx': 'GPX import',
+  'undo.importGoogleList': 'Google Maps import',
+
 }
 
 export default nl