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/prestigePanel.tsx

@@ -15,6 +15,7 @@ import {
   PRESTIGE_UPGRADES,
   PRESTIGE_UPGRADE_CATEGORY_LABELS,
 } from "../../data/prestigeUpgrades.js";
+import { cdnImage } from "../../utils/cdn.js";
 import { sendNotification } from "../../utils/notification.js";
 import { playSound } from "../../utils/sound.js";
 import type { PrestigeUpgradeCategory } from "@elysium/types";
@@ -366,6 +367,11 @@ const PrestigePanel = (): JSX.Element => {
                           : ""}`}
                         key={upgrade.id}
                       >
+                        <img
+                          alt={upgrade.name}
+                          className="card-thumbnail"
+                          src={cdnImage("prestige-upgrades", upgrade.id)}
+                        />
                         <div className="shop-upgrade-info">
                           <h4>{upgrade.name}</h4>
                           <p>{upgrade.description}</p>