Add combatant side assignment for encounter difficulty

Combatants can now be assigned to party or enemy side via a toggle
in the difficulty breakdown panel. Party-side NPCs subtract their XP
from the encounter total, letting allied NPCs reduce difficulty.
PCs default to party, non-PCs to enemy — users who don't use sides
see no change. Side persists across reload and export/import.

Closes #22

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

apps/web/src/hooks/use-encounter.ts

@@ -12,6 +12,7 @@ import {
 	setCrUseCase,
 	setHpUseCase,
 	setInitiativeUseCase,
+	setSideUseCase,
 	setTempHpUseCase,
 	toggleConcentrationUseCase,
 	toggleConditionUseCase,
@@ -54,6 +55,7 @@ type EncounterAction =
 	| { type: "set-temp-hp"; id: CombatantId; tempHp: number | undefined }
 	| { type: "set-ac"; id: CombatantId; value: number | undefined }
 	| { type: "set-cr"; id: CombatantId; value: string | undefined }
+	| { type: "set-side"; id: CombatantId; value: "party" | "enemy" }
 	| {
 			type: "toggle-condition";
 			id: CombatantId;
@@ -321,6 +323,7 @@ function dispatchEncounterAction(
 		| { type: "set-temp-hp" }
 		| { type: "set-ac" }
 		| { type: "set-cr" }
+		| { type: "set-side" }
 		| { type: "toggle-condition" }
 		| { type: "toggle-concentration" }
 	>,
@@ -364,6 +367,9 @@ function dispatchEncounterAction(
 		case "set-cr":
 			result = setCrUseCase(store, action.id, action.value);
 			break;
+		case "set-side":
+			result = setSideUseCase(store, action.id, action.value);
+			break;
 		case "toggle-condition":
 			result = toggleConditionUseCase(store, action.id, action.conditionId);
 			break;
@@ -506,6 +512,11 @@ export function useEncounter() {
 				dispatch({ type: "set-cr", id, value }),
 			[],
 		),
+		setSide: useCallback(
+			(id: CombatantId, value: "party" | "enemy") =>
+				dispatch({ type: "set-side", id, value }),
+			[],
+		),
 		toggleCondition: useCallback(
 			(id: CombatantId, conditionId: ConditionId) =>
 				dispatch({ type: "toggle-condition", id, conditionId }),