feat(tutorial): add simple project editor

[nighca]

Summary

• add the /simple/sprites/<sprite-name> editor route and a focused Simple Mode layout for tutorial projects
• make the Stage Viewer read-only in Simple Mode, show a hovered sprite-name action, and insert its name at the current code-editor selection
• add the CodeEditor API allowlist used by tutorials; it filters both API Reference and LSP completion entries while retaining non-API language snippets
• add the presentation-ready stage ruler UI and a fixed 16px Simple Mode editor font size
• support Course Spotlight with stable Radar selectors, mask, duration, and click-to-dismiss behavior
• add Save as my project, export the SPX project only (without course-local generated state), then navigate to the newly created owned project

Deferred scope

• The remaining Course framework capabilities remain coordinated with #3417. This PR wires Spotlight through the route-local Playground runner without duplicating that broader framework work.

Verification

• pnpm run type-check
• pnpm run lint
• pnpm exec vitest --run src/components/editor/editor-state.test.ts src/components/tutorials/playground/runner.test.ts src/components/xgo-code-editor/common.test.ts src/utils/spotlight/index.test.ts
• started the local Vite server and exercised the isolated built-in Tutorial mock project. It correctly normalizes /playground/simple to /playground/simple/sprites/Squirrel/code and renders the focused code editor, read-only preview, and Run control.

Closes #3416