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>

packages/domain/src/index.ts

@@ -71,6 +71,7 @@ export type {
 	TurnAdvanced,
 	TurnRetreated,
 } from "./events.js";
+export type { ExportBundle } from "./export-bundle.js";
 export { deriveHpStatus, type HpStatus } from "./hp-status.js";
 export {
 	calculateInitiative,