feat(08-01): install Recharts and create tested data aggregation functions

- Install recharts@3.8.1 as dependency
- Create chart-utils.ts with 4 exported pure functions: aggregateRunsByDay, aggregateCostsByDay, aggregateAgentWorkload, formatShortDate
- Create dashboard-charts.test.ts with 16 unit tests covering all aggregation behaviors
- All tests pass: 14-day bucketing with zero-fill, timed_out/interrupted as failed, null cost handling, top-8 + Others grouping, name truncation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -47,6 +47,7 @@
     "immer": "^11.1.4",
     "react": "19.2.4",
     "react-dom": "19.2.4",
+    "recharts": "3.8.1",
     "tailwind-merge": "3.5.0",
     "zod": "4.3.6",
     "zustand": "^5.0.12"