feat: implement personalized Neighborhood clustering for authenticated users

[Pruthviangadi]

This PR implements the "Neighborhood" feature, which personalizes the Git City layout for authenticated users by:

User Centering: The logged-in user's building is now prioritized and placed at the city's origin
(0, 0, 0)
.
Social Graph Clustering: We now fetch the user's GitHub followers and following who have joined Git City and cluster them in a dedicated neighborhood surrounding the user.
Dynamic Layout Engine: Updated
generateCityLayout
to seamlessly merge neighborhood connections into the city pool while preventing any duplicates or overlaps with standard "Downtown" or "District" buildings.
Real-time Integration: The frontend now dynamically triggers city regeneration as soon as social connection data is retrieved, ensuring immediate visual feedback upon login.
Related issue
Implements the community-requested "Neighborhoods" and "District Zoning" feature to provide a more personalized player experience.

Screenshots
(N/A - This PR provides the core backend API, layout algorithms, and frontend integration logic)

Checklist
npm run lint passes
Tested locally
No secrets or .env values committed

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