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/commands/inventory/inventory.ts

@@ -2,7 +2,8 @@ import { createCommand } from "@/lib/utils";
 import { SlashCommandBuilder } from "discord.js";
 import { inventoryService } from "@/modules/inventory/inventory.service";
 import { userService } from "@/modules/user/user.service";
-import { createWarningEmbed, createBaseEmbed } from "@lib/embeds";
+import { createWarningEmbed } from "@lib/embeds";
+import { getInventoryEmbed } from "@/modules/inventory/inventory.view";
 
 export const inventory = createCommand({
     data: new SlashCommandBuilder()
@@ -24,18 +25,19 @@ export const inventory = createCommand({
         }
 
         const user = await userService.getOrCreateUser(targetUser.id, targetUser.username);
-        const items = await inventoryService.getInventory(user.id);
+        if (!user) {
+            await interaction.editReply({ embeds: [createWarningEmbed("Failed to load user data.", "Error")] });
+            return;
+        }
+
+        const items = await inventoryService.getInventory(user.id.toString());
 
         if (!items || items.length === 0) {
             await interaction.editReply({ embeds: [createWarningEmbed("Inventory is empty.", `${user.username}'s Inventory`)] });
             return;
         }
 
-        const description = items.map(entry => {
-            return `**${entry.item.name}** x${entry.quantity}`;
-        }).join("\n");
-
-        const embed = createBaseEmbed(`${user.username}'s Inventory`, description, "Blue");
+        const embed = getInventoryEmbed(items, user.username);
 
         await interaction.editReply({ embeds: [embed] });
     }