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>

specs/007-json-import-export/quickstart.md

@@ -0,0 +1,52 @@
+# Quickstart: JSON Import/Export
+
+**Feature**: 007-json-import-export
+**Date**: 2026-03-27
+
+## Overview
+
+Export and import the full application state (encounter, undo/redo history, player characters) as a JSON file. Enables backup/restore and sharing encounters between DMs.
+
+## Key Concepts
+
+- **ExportBundle**: A JSON object containing `version`, `exportedAt`, `encounter`, `undoStack`, `redoStack`, and `playerCharacters`. This is the file format.
+- **Full replacement**: Import replaces all existing state — encounter, undo/redo, and player characters. It's not a merge.
+- **Validation reuse**: Import validation uses the same `rehydrateEncounter()` and player character validation functions that localStorage loading uses.
+
+## Implementation Layers
+
+### Domain Layer
+
+No new domain functions are needed. The existing types (`Encounter`, `PlayerCharacter`, `UndoRedoState`) and validation functions are reused as-is.
+
+A new `ExportBundle` type is defined in the domain layer as a pure data structure.
+
+### Application Layer
+
+A new use case for import validation and bundle assembly:
+- `validateImportBundle(data: unknown)` — validates and rehydrates an export bundle, returning the validated bundle or an error.
+
+Export assembly is straightforward enough to live in the adapter layer (it's just reading and packaging existing state).
+
+### Adapter Layer (Web)
+
+- **Export**: Read state from contexts, assemble bundle, trigger browser download via `URL.createObjectURL()` + anchor element.
+- **Import**: File picker input, parse JSON, delegate to application-layer validation, show confirmation dialog if encounter is non-empty, replace state via context setters.
+- **UI**: Two new overflow menu items in the action bar — "Export Encounter" and "Import Encounter".
+
+## File Locations
+
+| Artifact | Path |
+|----------|------|
+| ExportBundle type | `packages/domain/src/types.ts` or new file |
+| Import validation use case | `packages/application/src/` |
+| Export/import adapter functions | `apps/web/src/persistence/` |
+| UI integration | `apps/web/src/components/action-bar.tsx` |
+| Confirmation dialog | `apps/web/src/components/` (new or reuse existing confirm pattern) |
+
+## Testing Strategy
+
+- **Domain**: No new domain tests needed (existing types unchanged).
+- **Application**: Test `validateImportBundle()` with valid bundles, invalid bundles, missing fields, wrong types, and edge cases (empty encounter, empty stacks).
+- **Adapter**: Test export bundle assembly and import file handling.
+- **Integration**: Round-trip test — export then import should produce identical state.