bump typescript to v6

[rmens]

Summary

• TypeScript 5 → 6
• No code/config changes needed: SvelteKit's generated tsconfig already sets types: ["node"] and all other options explicitly
• Note: openapi-typescript has a peer dep on ^5.x but works fine with TS 6

Test plan

• Run npm run check locally
• Run npm run check-all

[rmens]

Blocked on upstream: openapi-typescript@7.x requires typescript@^5.x.

There's an open PR (openapi-ts/openapi-typescript#2711) to widen the peer dep to ^5.x || ^6.0.0, but it hasn't been merged yet. There's also a type-level breaking change in TS6 where Date extends object evaluates to true, which breaks their Readable<T>/Writable<T> helpers (openapi-ts/openapi-typescript#2723).

Parking this until upstream ships TS6 support.