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>

CLAUDE.md

@@ -7,6 +7,41 @@
 2. `pnpm build` — all packages build cleanly
 3. `pnpm test` — all tests pass with 100% coverage on `apps/api` and `packages/types`
 
+## Art Assets
+
+Game art is generated via the Gemini API (`gemini-3-pro-image-preview`, ~$0.134/image at 1K resolution) and hosted on the CDN at `https://cdn.nhcarrigan.com/elysium/`.
+
+### Process
+1. Generate images with `curl` to `https://generativelanguage.googleapis.com/v1beta/models/gemini-3-pro-image-preview:generateContent?key=<API_KEY>`, requesting soft-shaded anime style
+2. Save responses to `/home/naomi/code/naomi/elysium/img/<category>/<id>.jpg`
+3. Upload to R2 with: `AWS_ACCESS_KEY_ID=dd0a3d73969143ada84d50f8940cc5e2 AWS_SECRET_ACCESS_KEY=f73e9907da1b2297e93e17f786d6446d33d4ac60e185879578a0d5020899b18e aws s3 sync img/ s3://nhcarrigan-cdn/elysium/ --endpoint-url https://751c386661d378cc032093493cfb0869.r2.cloudflarestorage.com`
+4. Delete the local `img/` directory before committing (images live on CDN only)
+
+### CDN URL Helper
+`apps/web/src/utils/cdn.ts` exports `cdnImage(folder, id)` → `https://cdn.nhcarrigan.com/elysium/<folder>/<id>.jpg`
+
+### Directory → Category Mapping
+| Game entity | CDN folder |
+|---|---|
+| Zones | `zones` |
+| Bosses | `bosses` |
+| Quests | `quests` |
+| Adventurers | `adventurers` |
+| Companions | `companions` |
+| Equipment | `equipment` |
+| Upgrades | `upgrades` |
+| Prestige upgrades | `prestige-upgrades` |
+| Transcendence upgrades | `transcendence-upgrades` |
+| Achievements | `achievements` |
+| Explorations | `explorations` |
+| Materials | `materials` |
+| Recipes | `recipes` |
+| Story chapter banners | `story-chapters` |
+
+### API Rate Limits
+- 250 images/day per API key — use a second key if quota is hit
+- Free-tier keys cannot use `gemini-3-pro-image-preview`; key must be on a billing-linked project
+
 ## About Page
 
 The About page (`apps/web/src/components/game/aboutPanel.tsx`) contains a **How to Play** guide that should be kept up to date as new features are added to the game. When implementing new game systems, zones, mechanics, or significant UI features, update the `HOW_TO_PLAY` array in `aboutPanel.tsx` to include a description of the new feature.