Fix GitHub preview repo selection and Three.js bar tooltip vertical alignment

[Copilot]

The preview modal was not using the intended repository subset for the GitHub project visualization, and the Three.js bar chart tooltip appeared detached from bar tops. This PR scopes preview graph input correctly and adjusts tooltip positioning to track rendered bar height.

• Preview modal: use correct repo source for project graph

  • In PortfolioPreviewModal, stopped discarding the repos prop and now build preview graph data from the preview subset.
  • Added fallback to allRepos when preview subset is empty to preserve behavior for edge cases.

• Three.js bar chart: correct tooltip Y anchor

  • In ThreeBarChart, updated tooltip anchor calculation so the tooltip sits near the visible bar top instead of floating too high.
  • Replaced inline offset literal with a named constant for readability and maintenance.

// PortfolioPreviewModal.tsx
const previewRepos = buildRepos(repos.length > 0 ? repos : allRepos)
...
<GitHubSection repos={previewRepos} />

// ThreeBarChart.tsx
const TOOLTIP_TOP_OFFSET = 0.1
...
pos3.y = bars[hoveredIdx].mesh.scale.y + TOOLTIP_TOP_OFFSET