Add JSON import/export for full encounter state

Export and import encounter, undo/redo history, and player characters
as a downloadable .json file. Export/import actions are in the action
bar overflow menu. Import validates using existing rehydration functions
and shows a confirmation dialog when replacing a non-empty encounter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/use-player-characters.ts

@@ -103,6 +103,7 @@ export function usePlayerCharacters() {
 		createCharacter,
 		editCharacter,
 		deleteCharacter,
+		replacePlayerCharacters: setCharacters,
 		makeStore,
 	} as const;
 }