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/persistence/player-character-storage.ts

@@ -22,7 +22,7 @@ function isValidOptionalMember(
 	return value === undefined || (typeof value === "string" && valid.has(value));
 }
 
-function rehydrateCharacter(raw: unknown): PlayerCharacter | null {
+export function rehydrateCharacter(raw: unknown): PlayerCharacter | null {
 	if (typeof raw !== "object" || raw === null || Array.isArray(raw))
 		return null;
 	const entry = raw as Record<string, unknown>;