feat: integrate art assets across all game panels (#43)

## Summary

- Adds `apps/web/src/utils/cdn.ts` with a `cdnImage(folder, id)` helper that builds URLs from `https://cdn.nhcarrigan.com/elysium/`
- Wires CDN art into all 13 game panels (bosses, quests, adventurers, companions, equipment, upgrades, prestige, transcendence, achievements, explorations, crafting, story, codex)
- Zone selector tabs now display 16:9 zone art thumbnails in place of emoji icons
- Adds a fixed background image at 15% opacity via `body::before`
- Documents the art generation and CDN upload process in `CLAUDE.md` for future expansions

Resolves #15

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #43
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/components/game/achievementPanel.tsx

@@ -7,6 +7,7 @@
 /* eslint-disable react/no-multi-comp -- Sub-component is tightly coupled to this panel */
 import { type JSX, useState } from "react";
 import { useGame } from "../../context/gameContext.js";
+import { cdnImage } from "../../utils/cdn.js";
 import { LockToggle } from "../ui/lockToggle.js";
 import type { Achievement } from "@elysium/types";
 
@@ -76,7 +77,11 @@ const AchievementCard = ({
     <div className={`achievement-card ${isUnlocked
       ? "unlocked"
       : "locked"}`}>
-      <div className="achievement-icon">{achievement.icon}</div>
+      <img
+        alt={achievement.name}
+        className="card-thumbnail"
+        src={cdnImage("achievements", achievement.id)}
+      />
       <div className="achievement-info">
         <h3>{achievement.name}</h3>
         <p>{achievement.description}</p>