chore(deps): align website deps with Dependabot; add TS 6/7 split

Bump the workshop site dependencies to the versions Dependabot proposed,
superseding the individual bump PRs:

- astro 5.16 -> 7.0.9
- @astrojs/starlight 0.37 -> 0.41.3 (peers astro ^7, moves with astro)
- sharp 0.34 -> 0.35.3
- @astrojs/check 0.9.4 -> 0.9.9
- typescript 5.7 -> 6.0.3

TypeScript is held on the 6.x line because @astrojs/check (our type-checker)
peers `typescript@^5 || ^6` and does not yet support TypeScript 7. To still
exercise the TS 7 toolchain, add a split: `@typescript/native-preview` (tsgo)
as a dev dependency, a dedicated `tsconfig.tsgo.json`, and `check:tsgo` /
`check:all` scripts. `npm run check` runs astro check (TS 6); `check:tsgo`
runs tsgo (TS 7); `check:all` runs both.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 64357866-3a3f-4fd8-914b-77826de5be5a

Author: GeekTrainer