Add CI build with private dependency mocks

[Frotty]

What changed

• Add a GitHub Actions build workflow for install, lint, typecheck, tests, bundling, and VSIX validation.
• Replace private local casc-ts and war3-model checkouts in CI with committed API/fixture mocks.
• Keep real image fixture coverage for local development while allowing mock-mode CI tests.
• Prewarm model thumbnails when the model asset browser opens or the model tab is selected.
• Refresh the lockfile dependencies.

Why

The extension consumes casc-ts and war3-model through local file:../... dependencies, but those repositories are not publicly checkoutable in GitHub Actions. The committed mocks preserve the expected sibling layout and enough API/shader/model behavior for CI to validate the extension itself without exposing private sources.

Validation

• npm run lint
• npx tsc -p . --noEmit
• npm test
• mock-mode npm test
• npm run package-web
• npm run test:vsix-contents