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>

apps/web/src/components/__tests__/turn-navigation.test.tsx

@@ -54,6 +54,10 @@ function mockContext(overrides: Partial<Encounter> = {}) {
 		addFromBestiary: vi.fn(),
 		addFromPlayerCharacter: vi.fn(),
 		makeStore: vi.fn(),
+		undo: vi.fn(),
+		redo: vi.fn(),
+		canUndo: false,
+		canRedo: false,
 		events: [],
 	};