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/styles.css

@@ -33,6 +33,20 @@ body {
   color: var(--colour-text);
   font-family: var(--font);
   min-height: 100vh;
+  position: relative;
+}
+
+body::before {
+  background-attachment: fixed;
+  background-image: url("https://cdn.nhcarrigan.com/elysium/background.jpg");
+  background-position: center;
+  background-size: cover;
+  content: "";
+  inset: 0;
+  opacity: 0.15;
+  pointer-events: none;
+  position: fixed;
+  z-index: -1;
 }
 
 /* ===================== RESOURCE BAR ===================== */
@@ -2056,8 +2070,11 @@ body {
   opacity: 0.45;
 }
 
-.zone-emoji {
-  font-size: 1.4rem;
+.zone-tab-image {
+  aspect-ratio: 16 / 9;
+  border-radius: 0.35rem;
+  object-fit: cover;
+  width: 96px;
 }
 
 .zone-name {
@@ -4465,3 +4482,28 @@ body {
   font-size: 0.8rem;
   line-height: 1.5;
 }
+
+/* ===================== CDN ASSET IMAGES ===================== */
+.card-thumbnail {
+  border-radius: var(--radius);
+  flex-shrink: 0;
+  height: 72px;
+  object-fit: cover;
+  width: 72px;
+}
+
+.story-chapter-banner {
+  border-radius: var(--radius);
+  height: 220px;
+  margin-bottom: 1rem;
+  object-fit: cover;
+  width: 100%;
+}
+
+.codex-entry-image {
+  border-radius: var(--radius);
+  height: 80px;
+  margin-bottom: 0.5rem;
+  object-fit: cover;
+  width: 80px;
+}