Add 'social_github' as a new attribution source option in the
onboarding 'How did you hear about Multica?' multi-select picker,
alongside the existing X / LinkedIn / YouTube options.
Includes:
- New 'social_github' value in the Source type union
- New GitHubIcon in the brand-icons component
- New option in step-source.tsx (placed next to other social picks)
- en/zh-Hans/ko i18n labels
Co-authored-by: Lambda <lambda@multica.ai>
Co-authored-by: multica-agent <github@multica.ai>
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