refactor: add inventory view layer

Create inventory.view.ts with UI logic extracted from commands:
- getInventoryEmbed() for inventory display
- getItemUseResultEmbed() for item use results

Updated commands with proper type safety:
- inventory.ts: add null check, convert user.id to string
- use.ts: add null check, convert user.id to string

Improves separation of concerns and type safety.

src/modules/inventory/inventory.view.ts

@@ -0,0 +1,40 @@
+import { EmbedBuilder } from "discord.js";
+import type { ItemUsageData } from "@/lib/types";
+
+/**
+ * Inventory entry with item details
+ */
+interface InventoryEntry {
+    quantity: bigint | null;
+    item: {
+        id: number;
+        name: string;
+        [key: string]: any;
+    };
+}
+
+/**
+ * Creates an embed displaying a user's inventory
+ */
+export function getInventoryEmbed(items: InventoryEntry[], username: string): EmbedBuilder {
+    const description = items.map(entry => {
+        return `**${entry.item.name}** x${entry.quantity}`;
+    }).join("\n");
+
+    return new EmbedBuilder()
+        .setTitle(`📦 ${username}'s Inventory`)
+        .setDescription(description)
+        .setColor(0x3498db); // Blue
+}
+
+/**
+ * Creates an embed showing the results of using an item
+ */
+export function getItemUseResultEmbed(results: string[], itemName?: string): EmbedBuilder {
+    const description = results.map(r => `• ${r}`).join("\n");
+
+    return new EmbedBuilder()
+        .setTitle("✅ Item Used!")
+        .setDescription(description)
+        .setColor(0x2ecc71); // Green/Success
+}