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/__tests__/export-import.test.ts

@@ -234,6 +234,57 @@ describe("round-trip: export then import", () => {
 		expect(imported.encounter.combatants[0].cr).toBe("2");
 	});
 
+	it("round-trips a combatant with side field", () => {
+		const encounterWithSide: Encounter = {
+			combatants: [
+				{
+					id: combatantId("c-1"),
+					name: "Allied Guard",
+					cr: "2",
+					side: "party",
+				},
+				{
+					id: combatantId("c-2"),
+					name: "Goblin",
+					side: "enemy",
+				},
+			],
+			activeIndex: 0,
+			roundNumber: 1,
+		};
+		const emptyUndoRedo: UndoRedoState = {
+			undoStack: [],
+			redoStack: [],
+		};
+		const bundle = assembleExportBundle(encounterWithSide, emptyUndoRedo, []);
+		const serialized = JSON.parse(JSON.stringify(bundle));
+		const result = validateImportBundle(serialized);
+
+		expect(typeof result).toBe("object");
+		const imported = result as ExportBundle;
+		expect(imported.encounter.combatants[0].side).toBe("party");
+		expect(imported.encounter.combatants[1].side).toBe("enemy");
+	});
+
+	it("round-trips a combatant without side field as undefined", () => {
+		const encounterNoSide: Encounter = {
+			combatants: [{ id: combatantId("c-1"), name: "Custom" }],
+			activeIndex: 0,
+			roundNumber: 1,
+		};
+		const emptyUndoRedo: UndoRedoState = {
+			undoStack: [],
+			redoStack: [],
+		};
+		const bundle = assembleExportBundle(encounterNoSide, emptyUndoRedo, []);
+		const serialized = JSON.parse(JSON.stringify(bundle));
+		const result = validateImportBundle(serialized);
+
+		expect(typeof result).toBe("object");
+		const imported = result as ExportBundle;
+		expect(imported.encounter.combatants[0].side).toBeUndefined();
+	});
+
 	it("round-trips an empty encounter", () => {
 		const emptyEncounter: Encounter = {
 			combatants: [],