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/turn-navigation.tsx

@@ -1,11 +1,19 @@
-import { StepBack, StepForward, Trash2 } from "lucide-react";
+import { Redo2, StepBack, StepForward, Trash2, Undo2 } from "lucide-react";
 import { useEncounterContext } from "../contexts/encounter-context.js";
 import { Button } from "./ui/button.js";
 import { ConfirmButton } from "./ui/confirm-button.js";
 
 export function TurnNavigation() {
-	const { encounter, advanceTurn, retreatTurn, clearEncounter } =
-		useEncounterContext();
+	const {
+		encounter,
+		advanceTurn,
+		retreatTurn,
+		clearEncounter,
+		undo,
+		redo,
+		canUndo,
+		canRedo,
+	} = useEncounterContext();
 
 	const hasCombatants = encounter.combatants.length > 0;
 	const isAtStart = encounter.roundNumber === 1 && encounter.activeIndex === 0;
@@ -24,6 +32,29 @@ export function TurnNavigation() {
 				<StepBack className="h-5 w-5" />
 			</Button>
 
+			<div className="flex items-center gap-1">
+				<Button
+					variant="ghost"
+					size="icon"
+					onClick={undo}
+					disabled={!canUndo}
+					title="Undo"
+					aria-label="Undo"
+				>
+					<Undo2 className="h-4 w-4" />
+				</Button>
+				<Button
+					variant="ghost"
+					size="icon"
+					onClick={redo}
+					disabled={!canRedo}
+					title="Redo"
+					aria-label="Redo"
+				>
+					<Redo2 className="h-4 w-4" />
+				</Button>
+			</div>
+
 			<div className="flex min-w-0 flex-1 items-center justify-center gap-2 text-sm">
 				<span className="shrink-0 rounded-md bg-muted px-2 py-0.5 font-semibold text-foreground text-sm">
 					<span className="-mt-[3px] inline-block">