Refactor XR Keyboard layout and UI rendering

[jolicaD]

Refactor: Keyboard Layout & Rendering Engine

This PR modernizes the virtual keyboard by refactoring the layout logic and improving render stability in XR.

Key Changes

• Unified Layout Engine: Replaced manual weight calculations with a consolidated addKey method and constant-driven spacing (COL_SPACER, ROW_SPACER).

• Improved Depth Management: Added a recursive applyZOrder function to ensure all keyboard sub-elements render correctly on top of other world objects.

• Visual Polish: Switched to a borderless shader and updated the color palette for better legibility.

[google-cla]

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[ruofeidu]

Thank you Jolica! Would you Gemini solutions to update branch, sign CLA, and pass all prechecks?

Thanks!

[ruofeidu]

Jolica: Please remove .vscode/launch.json and fix the GitHub checks before sending it out to reviews -- this is the journey to learn : )

[ruofeidu]

Lint and Format Check is broken, please run:

npm run lint # ESLint check
npm run format # Prettier format

[jolicaD]

Thanks so much, Ruofei!