chore: add scripts/generate-og-image.sh to regenerate the OG card

Reproducible ImageMagick script that rebuilds static/og-preview.jpg from the
headshot + theme colours (with serif-font fallbacks so it runs on other
machines). Exposed as `npm run og:image`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -14,7 +14,8 @@
 		"format": "prettier --write .",
 		"format:check": "prettier --check .",
 		"test": "vitest run",
-		"test:watch": "vitest"
+		"test:watch": "vitest",
+		"og:image": "./scripts/generate-og-image.sh"
 	},
 	"devDependencies": {
 		"@sveltejs/adapter-auto": "^7.0.0",