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/difficulty-breakdown-panel.tsx

@@ -0,0 +1,109 @@
+import type { DifficultyTier } from "@initiative/domain";
+import { useRef } from "react";
+import { useEncounterContext } from "../contexts/encounter-context.js";
+import { useClickOutside } from "../hooks/use-click-outside.js";
+import {
+	type BreakdownCombatant,
+	useDifficultyBreakdown,
+} from "../hooks/use-difficulty-breakdown.js";
+import { CrPicker } from "./cr-picker.js";
+
+const TIER_LABELS: Record<DifficultyTier, { label: string; color: string }> = {
+	trivial: { label: "Trivial", color: "text-muted-foreground" },
+	low: { label: "Low", color: "text-green-500" },
+	moderate: { label: "Moderate", color: "text-yellow-500" },
+	high: { label: "High", color: "text-red-500" },
+};
+
+function formatXp(xp: number): string {
+	return xp.toLocaleString();
+}
+
+function CombatantRow({ entry }: { entry: BreakdownCombatant }) {
+	const { setCr } = useEncounterContext();
+
+	const nameLabel = entry.source
+		? `${entry.combatant.name} (${entry.source})`
+		: entry.combatant.name;
+
+	return (
+		<div className="flex items-center justify-between gap-2 text-xs">
+			<span className="min-w-0 truncate" title={nameLabel}>
+				{nameLabel}
+			</span>
+			<div className="flex shrink-0 items-center gap-2">
+				{entry.editable ? (
+					<CrPicker
+						value={entry.cr}
+						onChange={(cr) => setCr(entry.combatant.id, cr)}
+					/>
+				) : (
+					<span className="text-muted-foreground">
+						{entry.cr ? `CR ${entry.cr}` : "—"}
+					</span>
+				)}
+				<span className="w-12 text-right tabular-nums">
+					{entry.xp == null ? "—" : formatXp(entry.xp)}
+				</span>
+			</div>
+		</div>
+	);
+}
+
+export function DifficultyBreakdownPanel({ onClose }: { onClose: () => void }) {
+	const ref = useRef<HTMLDivElement>(null);
+	useClickOutside(ref, onClose);
+
+	const breakdown = useDifficultyBreakdown();
+	if (!breakdown) return null;
+
+	const tierConfig = TIER_LABELS[breakdown.tier];
+
+	return (
+		<div
+			ref={ref}
+			className="absolute top-full right-0 z-50 mt-1 w-72 rounded-lg border border-border bg-card p-3 shadow-lg"
+		>
+			<div className="mb-2 font-medium text-sm">
+				Encounter Difficulty:{" "}
+				<span className={tierConfig.color}>{tierConfig.label}</span>
+			</div>
+
+			<div className="mb-2 border-border border-t pt-2">
+				<div className="mb-1 text-muted-foreground text-xs">
+					Party Budget ({breakdown.pcCount}{" "}
+					{breakdown.pcCount === 1 ? "PC" : "PCs"})
+				</div>
+				<div className="flex gap-3 text-xs">
+					<span>
+						Low: <strong>{formatXp(breakdown.partyBudget.low)}</strong>
+					</span>
+					<span>
+						Mod: <strong>{formatXp(breakdown.partyBudget.moderate)}</strong>
+					</span>
+					<span>
+						High: <strong>{formatXp(breakdown.partyBudget.high)}</strong>
+					</span>
+				</div>
+			</div>
+
+			<div className="border-border border-t pt-2">
+				<div className="mb-1 flex justify-between text-muted-foreground text-xs">
+					<span>Monsters</span>
+					<span>XP</span>
+				</div>
+				<div className="flex flex-col gap-1">
+					{breakdown.combatants.map((entry) => (
+						<CombatantRow key={entry.combatant.id} entry={entry} />
+					))}
+				</div>
+				<div className="mt-2 flex justify-between border-border border-t pt-2 font-medium text-xs">
+					<span>Total Monster XP</span>
+					<span className="tabular-nums">
+						{formatXp(breakdown.totalMonsterXp)}
+					</span>
+				</div>
+			</div>
+		</div>
+	);
+}