feat(02-03): add asset generation build script and generate all character assets

- Create scripts/generate-assets.ts: reads config.json, calls Gemini API for each companion
- Generate 4 character sheets (3-view: front, side, face) for Fee Lila, Einhorn Stella, Fuchs Finn, Eule Elsa
- Generate 4 avatars using character sheets as reference images for consistency
- Generate 1 style reference (initial forest scene) for art direction
- Add tsx devDependency and npm run generate-assets script
- Fix imageModel name: gemini-3.1-flash-preview-image -> gemini-3.1-flash-image-preview
- Script handles errors gracefully, skips already-generated assets, logs progress

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -10,11 +10,13 @@
 		"test": "vitest run",
 		"test:watch": "vitest",
 		"lint": "biome check .",
-		"lint:fix": "biome check --write ."
+		"lint:fix": "biome check --write .",
+		"generate-assets": "tsx scripts/generate-assets.ts"
 	},
 	"devDependencies": {
 		"@biomejs/biome": "^1.9.0",
 		"fake-indexeddb": "^6.2.5",
+		"tsx": "^4.0.0",
 		"typescript": "~5.8.0",
 		"vite": "^6.0.0",
 		"vitest": "^3.0.0"