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/adventurerPanel.tsx

@@ -9,18 +9,10 @@
 /* eslint-disable complexity -- Complex component with many render paths */
 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 { Adventurer } from "@elysium/types";
 
-const iconByClass: Record<string, string> = {
-  cleric:  "✝️",
-  mage:    "🔮",
-  paladin: "🛡️",
-  ranger:  "🏹",
-  rogue:   "🗝️",
-  warrior: "🗡️",
-};
-
 type BatchSize = 1 | 5 | 10 | 25 | 100 | "max";
 const batchOptions: Array<BatchSize> = [ 1, 5, 10, 25, 100, "max" ];
 
@@ -105,14 +97,15 @@ const AdventurerCard = ({
     ? `🪙 ${formatNumber(Math.ceil(cost))}${maxSuffix}`
     : "🔒 Locked";
 
-  // eslint-disable-next-line @typescript-eslint/dot-notation -- "class" is a reserved word
-  const adventurerIcon = iconByClass[adventurer["class"]] ?? "⚔️";
-
   return (
     <div className={`adventurer-card ${adventurer.unlocked
       ? ""
       : "locked"}`}>
-      <div className="adventurer-icon">{adventurerIcon}</div>
+      <img
+        alt={adventurer.name}
+        className="card-thumbnail"
+        src={cdnImage("adventurers", adventurer.id)}
+      />
       <div className="adventurer-info">
         <h3>{adventurer.name}</h3>
         <p>