budget2.0/docs/plans/2026-09-30-session-undo-redo.md

Session undo and redo implementation

1. Extend snapshot-store tests with changed and unchanged edits, undo/redo ordering, a new edit after undo, the 50-step bound, queued writes, failed writes, import/reset boundaries, and session reload behavior. Run them red.
2. Add history state and queued undo/redo operations in snapshot.ts. Ensure history changes only after persistence commits. Run the store tests green.
3. Add header controls and keyboard handling with an interaction test for disabled states, error display, and native editable shortcuts. Run it red, implement, then run it green.
4. Update architecture and user-facing documentation. Run bun test, bunx tsc --noEmit, build, and inspect desktop/mobile browser captures.