feat(shared-ui): add unified statistics components with theme support

- Add reusable chart components in shared-ui (StatsGrid, ActivityHeatmap, TrendLineChart, DonutChart, ProgressBars, StatisticsSkeleton)
- Use CSS variables (--primary) for consistent theme-based styling
- Add statistics pages to Calendar and Contacts apps
- Add statistics stores with app-specific metrics
- Fix PriorityDonutChart layout in Todo app (vertical layout with 2x2 legend grid)
- Add date-fns dependency to shared-ui

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pnpm-lock.yaml

@@ -4298,6 +4298,9 @@ importers:
       d3-zoom:
         specifier: ^3.0.0
         version: 3.0.0
+      date-fns:
+        specifier: ^4.1.0
+        version: 4.1.0
       lucide-svelte:
         specifier: ^0.468.0
         version: 0.468.0(svelte@5.44.0)