[codex] Implement Epic 6 Blockly editor

[PilHliP211]

Summary

Implements Epic 6: the Blockly-powered block editor foundation for CodeQuest.

• Adds pinned blockly@12.5.1 and Vite dependency optimization.
• Adds BlockEditor, RunButton, block registration, toolbox generation, and CodeQuest Blockly theme modules under src/editor/.
• Adds focused unit/component tests for registry behavior, toolbox filtering, run button state, code generation, and workspace serialization.
• Applies Blockly pixel rendering CSS and limits Vitest discovery to src/ tests so local worktree copies are not collected.
• Marks all Epic 6 story checklists complete in docs/stories/E-06/.

Notes

Manual browser smoke testing caught a Blockly 12 runtime issue: blocks cannot define both colour and style. The registry now strips the content-pack colour field when applying category styles so the toolbox categories and blocks render correctly.

Validation

• ESLint passed under Node 22.
• tsc --noEmit passed under Node 22.
• Vitest passed under Node 22: 18 files, 96 tests.
• Prettier check passed.
• Vite production build passed.
• Temporary Chrome/Playwright smoke test verified categories render, a block can be dragged into the workspace, Run generates moveEast(), and workspace state serializes.

Environment

The repo requires Node 22. The default shell Node 20 environment hit a jsdom ESM boundary, so full validation was run with Node v22.22.2.