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/transcendencePanel.tsx

@@ -7,12 +7,14 @@
 /* eslint-disable max-lines-per-function -- Complex component with many render paths */
 /* eslint-disable complexity -- Many conditional render paths */
 /* eslint-disable max-statements -- Transcendence panel manages many local state variables */
+/* eslint-disable max-lines -- Transcendence panel with CDN images exceeds line limit */
 import { useState, type JSX } from "react";
 import { useGame } from "../../context/gameContext.js";
 import {
   TRANSCENDENCE_UPGRADES,
   TRANSCENDENCE_UPGRADE_CATEGORY_LABELS,
 } from "../../data/transcendenceUpgrades.js";
+import { cdnImage } from "../../utils/cdn.js";
 import type { TranscendenceUpgradeCategory } from "@elysium/types";
 
 const echoFormulaConstant = 853;
@@ -301,6 +303,11 @@ const TranscendencePanel = (): JSX.Element => {
                           : ""}`}
                         key={upgrade.id}
                       >
+                        <img
+                          alt={upgrade.name}
+                          className="card-thumbnail"
+                          src={cdnImage("transcendence-upgrades", upgrade.id)}
+                        />
                         <div className="shop-upgrade-info">
                           <h4>{upgrade.name}</h4>
                           <p>{upgrade.description}</p>