Restructure dashboard layout: KPIs up top + add commit trend chart

[yunwei-zh]

Summary

• Moves dashboard KPI cards to the top of the main content so key stats are visible earlier (reduces “bottom-heavy” feel).
• Wires the previously-unused CommitTrendChart into the dashboard and places it alongside LeaderboardCharts on large screens (stacked on mobile).
• Improves information hierarchy and density without any backend/API changes.

Related Issues

Closes #146

Changes

• Dashboard layout
  • KPI grid is now rendered above charts/tables (above-the-fold on most screens).
  • CommitTrendChart is now imported and rendered on the dashboard.
  • Responsive 2-column section on lg+: LeaderboardCharts + CommitTrendChart; stacks on smaller breakpoints.
• Copy tweak
  • KPI label changed from “Total Lines Committed” to “Lines Changed” (same totalLinesChanged value) to avoid over- emphasizing raw line counts.

Test

• Testing
  • npm run build (passes)
  • Manual smoke check:
  • Dashboard loads and renders KPI cards near the top
  • Trend chart renders and remains usable on mobile/desktop breakpoints
  • Layout remains stable with the right sidebar on xl screens

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Screenshots