Add manual CR assignment and difficulty breakdown panel

Implement issue #21: custom combatants can now have a challenge rating
assigned via a new breakdown panel, opened by tapping the difficulty
indicator. Bestiary-linked combatants show read-only CR with source name;
custom combatants get a CR picker with all standard 5e values. CR persists
across reloads and round-trips through JSON export/import.

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

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

@@ -1,6 +1,8 @@
 import { Redo2, StepBack, StepForward, Trash2, Undo2 } from "lucide-react";
+import { useState } from "react";
 import { useEncounterContext } from "../contexts/encounter-context.js";
 import { useDifficulty } from "../hooks/use-difficulty.js";
+import { DifficultyBreakdownPanel } from "./difficulty-breakdown-panel.js";
 import { DifficultyIndicator } from "./difficulty-indicator.js";
 import { Button } from "./ui/button.js";
 import { ConfirmButton } from "./ui/confirm-button.js";
@@ -18,6 +20,7 @@ export function TurnNavigation() {
 	} = useEncounterContext();
 
 	const difficulty = useDifficulty();
+	const [showBreakdown, setShowBreakdown] = useState(false);
 	const hasCombatants = encounter.combatants.length > 0;
 	const isAtStart = encounter.roundNumber === 1 && encounter.activeIndex === 0;
 	const activeCombatant = encounter.combatants[encounter.activeIndex];
@@ -69,7 +72,19 @@ export function TurnNavigation() {
 				) : (
 					<span className="text-muted-foreground">No combatants</span>
 				)}
-				{difficulty && <DifficultyIndicator result={difficulty} />}
+				{difficulty && (
+					<div className="relative">
+						<DifficultyIndicator
+							result={difficulty}
+							onClick={() => setShowBreakdown((prev) => !prev)}
+						/>
+						{showBreakdown ? (
+							<DifficultyBreakdownPanel
+								onClose={() => setShowBreakdown(false)}
+							/>
+						) : null}
+					</div>
+				)}
 			</div>
 
 			<div className="flex flex-shrink-0 items-center gap-3">