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/it.ts

@@ -1485,6 +1485,21 @@ const it: Record<string, string | { name: string; category: string }[]> = {
   'perm.actionHint.packing_edit': 'Chi può gestire articoli da bagaglio e borse',
   'perm.actionHint.collab_edit': 'Chi può creare note, sondaggi e inviare messaggi',
   'perm.actionHint.share_manage': 'Chi può creare o eliminare link di condivisione pubblici',
+
+  // Undo
+  'undo.button': 'Annulla',
+  'undo.tooltip': 'Annulla: {action}',
+  'undo.assignPlace': 'Luogo assegnato al giorno',
+  'undo.removeAssignment': 'Luogo rimosso dal giorno',
+  'undo.reorder': 'Luoghi riordinati',
+  'undo.optimize': 'Percorso ottimizzato',
+  'undo.deletePlace': 'Luogo eliminato',
+  'undo.moveDay': 'Luogo spostato in altro giorno',
+  'undo.lock': 'Blocco luogo modificato',
+  'undo.importGpx': 'Importazione GPX',
+  'undo.importGoogleList': 'Importazione Google Maps',
+  'undo.addPlace': 'Luogo aggiunto',
+  'undo.done': 'Annullato: {action}',
 }
 
 export default it