Add undo/redo for all encounter actions

Memento-based undo/redo with full encounter snapshots. Undo stack
capped at 50 entries, persisted to localStorage. Triggered via
buttons in the top bar (inboard of turn navigation) and keyboard
shortcuts (Ctrl+Z / Ctrl+Shift+Z, Cmd on Mac, case-insensitive key
matching). Clear encounter resets both stacks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -8,6 +8,7 @@ A local-first initiative tracker and encounter manager for tabletop RPGs (D&D 5e
 - **Encounter state** — HP, AC, conditions, concentration tracking with visual status indicators
 - **Bestiary integration** — import bestiary JSON sources, search creatures, and view full stat blocks
 - **Player characters** — create reusable player character templates with name, AC, HP, color, and icon; search and add them to encounters with pre-filled stats; manage (edit/delete) from a dedicated panel
+- **Undo/redo** — reverse any encounter action with Undo/Redo buttons or keyboard shortcuts (Ctrl+Z / Ctrl+Shift+Z, Cmd on Mac); history persists across page reloads
 - **Persistent** — encounters survive page reloads via localStorage; bestiary data cached in IndexedDB; player characters stored independently
 
 ## Prerequisites