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/upgradePanel.tsx

@@ -9,6 +9,7 @@
 /* eslint-disable complexity -- UpgradeCard has many conditional render paths for states */
 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 { Upgrade } from "@elysium/types";
 
@@ -53,6 +54,11 @@ const UpgradeCard = ({
   if (upgrade.unlocked && upgrade.purchased) {
     return (
       <div className="upgrade-card purchased">
+        <img
+          alt={upgrade.name}
+          className="card-thumbnail"
+          src={cdnImage("upgrades", upgrade.id)}
+        />
         <span className="upgrade-name">
           {"✅ "}
           {upgrade.name}
@@ -65,6 +71,11 @@ const UpgradeCard = ({
   if (upgrade.unlocked) {
     return (
       <div className="upgrade-card">
+        <img
+          alt={upgrade.name}
+          className="card-thumbnail"
+          src={cdnImage("upgrades", upgrade.id)}
+        />
         <div className="upgrade-info">
           <h3>{upgrade.name}</h3>
           <p>{upgrade.description}</p>
@@ -108,6 +119,11 @@ const UpgradeCard = ({
 
   return (
     <div className="upgrade-card locked">
+      <img
+        alt={upgrade.name}
+        className="card-thumbnail"
+        src={cdnImage("upgrades", upgrade.id)}
+      />
       <div className="upgrade-info">
         <h3>
           {"🔒 "}