build: use pnpm exec instead of npx for vite-node in prebuild

Fixes build failure on server where system npm has a broken
lru-cache/Yallist dependency. Also adds vite-node as an explicit
dev dependency so it's always available via pnpm.

package.json

@@ -5,7 +5,7 @@
 	"type": "module",
 	"scripts": {
 		"dev": "vite dev",
-		"prebuild": "bash scripts/subset-emoji-font.sh && npx vite-node scripts/generate-mystery-verses.ts",
+		"prebuild": "bash scripts/subset-emoji-font.sh && pnpm exec vite-node scripts/generate-mystery-verses.ts",
 		"build": "vite build",
 		"preview": "vite preview",
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
@@ -39,6 +39,7 @@
 		"tslib": "^2.6.0",
 		"typescript": "^5.1.6",
 		"vite": "^7.1.3",
+		"vite-node": "^5.3.0",
 		"vitest": "^4.0.10"
 	},
 	"dependencies": {