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/codexPanel.tsx

@@ -8,6 +8,7 @@
 import { type JSX, useState } from "react";
 import { useGame } from "../../context/gameContext.js";
 import { CODEX_ENTRIES, ZONE_LABELS } from "../../data/codex.js";
+import { cdnImage } from "../../utils/cdn.js";
 import type { CodexEntry } from "@elysium/types";
 
 /**
@@ -36,6 +37,18 @@ const sourceBadge: Record<CodexEntry["sourceType"], string> = {
   zone:        "🗺️",
 };
 
+const sourceTypeFolder: Record<CodexEntry["sourceType"], string> = {
+  adventurer:  "adventurers",
+  boss:        "bosses",
+  equipment:   "equipment",
+  exploration: "explorations",
+  prestige:    "prestige-upgrades",
+  quest:       "quests",
+  recipe:      "recipes",
+  upgrade:     "upgrades",
+  zone:        "zones",
+};
+
 /**
  * Renders the codex panel with lore entries grouped by zone.
  * @returns The JSX element.
@@ -155,7 +168,17 @@ const CodexPanel = (): JSX.Element => {
                       </span>
                     </div>
                     {isExpanded
-                      ? <p className="codex-entry-content">{entry.content}</p>
+                      ? <>
+                        <img
+                          alt={entry.title}
+                          className="codex-entry-image"
+                          src={cdnImage(
+                            sourceTypeFolder[entry.sourceType],
+                            entry.sourceId,
+                          )}
+                        />
+                        <p className="codex-entry-content">{entry.content}</p>
+                      </>
                       : null}
                   </div>
                 );