Use TypeScript 5.7.x instead of 6.x

TypeScript 6.0 was available via npm install typescript but is still early-release; @cloudflare/workers-types and @cloudflare/vitest-pool-workers do not yet publish TS-6-compatible type declarations, causing tsc errors unrelated to the application code. TypeScript 5.7.x is the current stable LTS equivalent and is fully supported by all deps in this project. Pinned at ~5.7.0 in package.json; upgrade to TS 6 once the Cloudflare Workers types ecosystem catches up.