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/__tests__/use-difficulty-breakdown.test.tsx

@@ -106,7 +106,7 @@ describe("useDifficultyBreakdown", () => {
 		expect(result.current).toBeNull();
 	});
 
-	it("returns per-combatant entries with correct data", async () => {
+	it("returns per-combatant entries split by side", async () => {
 		const wrapper = makeWrapper({
 			encounter: buildEncounter({
 				combatants: [
@@ -145,29 +145,34 @@ describe("useDifficultyBreakdown", () => {
 			const breakdown = result.current;
 			expect(breakdown).not.toBeNull();
 			expect(breakdown?.pcCount).toBe(1);
-			// CR 1/4 = 50 + CR 2 = 450 → total 500
+			// CR 1/4 = 50 + CR 2 = 450 -> total 500
 			expect(breakdown?.totalMonsterXp).toBe(500);
-			expect(breakdown?.combatants).toHaveLength(3);
 
-			// Bestiary combatant
-			const goblin = breakdown?.combatants[0];
+			// PC in party column
+			expect(breakdown?.partyCombatants).toHaveLength(1);
+			expect(breakdown?.partyCombatants[0].combatant.name).toBe("Hero");
+			expect(breakdown?.partyCombatants[0].side).toBe("party");
+			expect(breakdown?.partyCombatants[0].level).toBe(5);
+
+			// Enemies: goblin, thug, bandit
+			expect(breakdown?.enemyCombatants).toHaveLength(3);
+
+			const goblin = breakdown?.enemyCombatants[0];
 			expect(goblin?.cr).toBe("1/4");
 			expect(goblin?.xp).toBe(50);
 			expect(goblin?.source).toBe("SRD");
 			expect(goblin?.editable).toBe(false);
+			expect(goblin?.side).toBe("enemy");
 
-			// Custom with CR
-			const thug = breakdown?.combatants[1];
+			const thug = breakdown?.enemyCombatants[1];
 			expect(thug?.cr).toBe("2");
 			expect(thug?.xp).toBe(450);
 			expect(thug?.source).toBeNull();
 			expect(thug?.editable).toBe(true);
 
-			// Custom without CR
-			const bandit = breakdown?.combatants[2];
+			const bandit = breakdown?.enemyCombatants[2];
 			expect(bandit?.cr).toBeNull();
 			expect(bandit?.xp).toBeNull();
-			expect(bandit?.source).toBeNull();
 			expect(bandit?.editable).toBe(true);
 		});
 	});
@@ -203,16 +208,15 @@ describe("useDifficultyBreakdown", () => {
 			wrapper,
 		});
 
-		// With no bestiary creatures loaded, the Ghost has null CR
 		const breakdown = result.current;
 		expect(breakdown).not.toBeNull();
-		const ghost = breakdown?.combatants[0];
+		const ghost = breakdown?.enemyCombatants[0];
 		expect(ghost?.cr).toBeNull();
 		expect(ghost?.xp).toBeNull();
 		expect(ghost?.editable).toBe(false);
 	});
 
-	it("excludes PC combatants from breakdown entries", async () => {
+	it("PC combatants appear in partyCombatants with level", async () => {
 		const wrapper = makeWrapper({
 			encounter: buildEncounter({
 				combatants: [
@@ -239,8 +243,53 @@ describe("useDifficultyBreakdown", () => {
 		});
 
 		await waitFor(() => {
-			expect(result.current?.combatants).toHaveLength(1);
-			expect(result.current?.combatants[0].combatant.name).toBe("Goblin");
+			expect(result.current?.partyCombatants).toHaveLength(1);
+			expect(result.current?.partyCombatants[0].combatant.name).toBe("Hero");
+			expect(result.current?.partyCombatants[0].level).toBe(1);
+			expect(result.current?.partyCombatants[0].side).toBe("party");
 		});
 	});
+
+	it("combatant with explicit side override is placed correctly", () => {
+		const wrapper = makeWrapper({
+			encounter: buildEncounter({
+				combatants: [
+					buildCombatant({
+						id: combatantId("c-1"),
+						name: "Hero",
+						playerCharacterId: pcId1,
+					}),
+					buildCombatant({
+						id: combatantId("c-2"),
+						name: "Allied Guard",
+						creatureId: goblinCreature.id,
+						side: "party",
+					}),
+					buildCombatant({
+						id: combatantId("c-3"),
+						name: "Thug",
+						cr: "1",
+					}),
+				],
+			}),
+			playerCharacters: [
+				{ id: pcId1, name: "Hero", ac: 15, maxHp: 30, level: 5 },
+			],
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		const { result } = renderHook(() => useDifficultyBreakdown(), {
+			wrapper,
+		});
+
+		const breakdown = result.current;
+		expect(breakdown).not.toBeNull();
+		// Allied Guard should be in party column
+		expect(breakdown?.partyCombatants).toHaveLength(2);
+		expect(breakdown?.partyCombatants[1].combatant.name).toBe("Allied Guard");
+		expect(breakdown?.partyCombatants[1].side).toBe("party");
+		// Thug in enemy column
+		expect(breakdown?.enemyCombatants).toHaveLength(1);
+		expect(breakdown?.enemyCombatants[0].combatant.name).toBe("Thug");
+	});
 });