fix: replace placeholder STL with real procedural mesh generator

[Kokila-chandrakar]

Description

Replace the fake placeholder STL download with a real procedural 3D mesh generator.

Previously, the "Download Printable 3D STL Monolith" button simulated a 1.2s
loading delay and downloaded a hardcoded single-triangle STL file, which was
not a valid or printable 3D model.

This PR implements Option A from the issue:

• Adds activityToTowers() in lib/export3d.ts to convert ActivityData[]
  into a TowerData[] grid, mapping contribution counts to mm heights
• Wires handleDownloadSTL in ShareSheet.tsx to call the real
  generateMonolithSTL() instead of the placeholder
• Adds 7 unit tests for activityToTowers in export3d.test.ts

Fixes #3676

Pillar

Bug Fix

Checklist

• My code follows the project's coding style
• I have added tests for my changes
• All existing tests pass
• I have updated relevant documentation
• My PR title follows the conventional commits format

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