feat: integrate CDN art assets across all game panels

Add cdnImage utility and wire zone/boss/quest/adventurer/companion/
equipment/upgrade/prestige/transcendence/achievement/exploration/
crafting/story panels to load art from cdn.nhcarrigan.com/elysium/.
Zone selector tabs display 16:9 zone art thumbnails. Background
image added as fixed overlay at 15% opacity. Document art generation
process in CLAUDE.md for future expansions.

Resolves #15

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>