feat: add session undo and redo

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

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+# 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.