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>

packages/domain/src/__tests__/rehydrate-combatant.test.ts

@@ -241,6 +241,28 @@ describe("rehydrateCombatant", () => {
 			expect(result?.cr).toBeUndefined();
 		});
 
+		it("preserves valid side field", () => {
+			for (const side of ["party", "enemy"]) {
+				const result = rehydrateCombatant({ ...minimalCombatant(), side });
+				expect(result).not.toBeNull();
+				expect(result?.side).toBe(side);
+			}
+		});
+
+		it("drops invalid side field", () => {
+			for (const side of ["ally", "", 42, null, true]) {
+				const result = rehydrateCombatant({ ...minimalCombatant(), side });
+				expect(result).not.toBeNull();
+				expect(result?.side).toBeUndefined();
+			}
+		});
+
+		it("combatant without side rehydrates as before", () => {
+			const result = rehydrateCombatant(minimalCombatant());
+			expect(result).not.toBeNull();
+			expect(result?.side).toBeUndefined();
+		});
+
 		it("drops invalid tempHp — keeps combatant", () => {
 			for (const tempHp of [-1, 1.5, "3"]) {
 				const result = rehydrateCombatant({