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>

src/renderer/components/dashboard/charts/chart-utils.ts

@@ -0,0 +1,142 @@
+import type { RunRecord, CostRecord, AgentRecord } from "@shared/types";
+
+export interface DailyRunData {
+  date: string;       // "YYYY-MM-DD"
+  total: number;
+  succeeded: number;
+  failed: number;
+}
+
+export interface DailyCostData {
+  date: string;       // "YYYY-MM-DD"
+  total: number;      // USD amount (NOT cents)
+}
+
+export interface AgentWorkloadData {
+  name: string;       // agent name, truncated to 10 chars
+  runs: number;
+}
+
+/**
+ * Aggregate runs into daily buckets for chart consumption.
+ * Returns `days` entries sorted ascending by date (oldest first).
+ * Days with no runs have {total:0, succeeded:0, failed:0}.
+ */
+export function aggregateRunsByDay(runs: RunRecord[], days: number): DailyRunData[] {
+  const now = new Date();
+  const buckets = new Map<string, { total: number; succeeded: number; failed: number }>();
+
+  // Initialize all day buckets with zero values
+  for (let i = 0; i < days; i++) {
+    const date = new Date(now.getTime() - i * 24 * 60 * 60 * 1000);
+    const key = date.toISOString().slice(0, 10);
+    buckets.set(key, { total: 0, succeeded: 0, failed: 0 });
+  }
+
+  // Fill buckets from actual data
+  for (const run of runs) {
+    const day = run.createdAt.slice(0, 10);
+    const bucket = buckets.get(day);
+    if (!bucket) continue;
+    bucket.total++;
+    if (run.status === "succeeded") bucket.succeeded++;
+    if (run.status === "failed" || run.status === "timed_out" || run.status === "interrupted") {
+      bucket.failed++;
+    }
+  }
+
+  // Return sorted ascending by date
+  return Array.from(buckets.entries())
+    .sort(([a], [b]) => a.localeCompare(b))
+    .map(([date, data]) => ({ date, ...data }));
+}
+
+/**
+ * Aggregate costs into daily buckets for chart consumption.
+ * Returns `days` entries sorted ascending by date.
+ * Days with no costs have {total:0}. Null amountUsd is treated as 0.
+ */
+export function aggregateCostsByDay(costs: CostRecord[], days: number): DailyCostData[] {
+  const now = new Date();
+  const buckets = new Map<string, number>();
+
+  // Initialize all day buckets with zero
+  for (let i = 0; i < days; i++) {
+    const date = new Date(now.getTime() - i * 24 * 60 * 60 * 1000);
+    const key = date.toISOString().slice(0, 10);
+    buckets.set(key, 0);
+  }
+
+  // Sum costs per day
+  for (const cost of costs) {
+    const day = cost.createdAt.slice(0, 10);
+    const current = buckets.get(day);
+    if (current === undefined) continue;
+    buckets.set(day, current + (cost.amountUsd ?? 0));
+  }
+
+  // Return sorted ascending by date
+  return Array.from(buckets.entries())
+    .sort(([a], [b]) => a.localeCompare(b))
+    .map(([date, total]) => ({ date, total }));
+}
+
+/**
+ * Group runs by agent and return workload data sorted by run count descending.
+ * If more than maxAgents, keep top maxAgents and aggregate rest into "Others".
+ * Agent names longer than 10 chars are truncated with ellipsis.
+ */
+export function aggregateAgentWorkload(
+  runs: RunRecord[],
+  agents: AgentRecord[],
+  maxAgents: number = 8,
+): AgentWorkloadData[] {
+  // Build name lookup from agents array
+  const nameMap = new Map<string, string>();
+  for (const agent of agents) {
+    nameMap.set(agent.id, agent.name);
+  }
+
+  // Count runs per agent
+  const counts = new Map<string, number>();
+  for (const run of runs) {
+    counts.set(run.agentId, (counts.get(run.agentId) ?? 0) + 1);
+  }
+
+  // Sort by count descending
+  const sorted = Array.from(counts.entries())
+    .sort(([, a], [, b]) => b - a);
+
+  const truncateName = (name: string): string =>
+    name.length > 10 ? name.slice(0, 10) + "..." : name;
+
+  // If within limit, return all
+  if (sorted.length <= maxAgents) {
+    return sorted.map(([agentId, runCount]) => ({
+      name: truncateName(nameMap.get(agentId) ?? agentId),
+      runs: runCount,
+    }));
+  }
+
+  // Split into top agents and "Others"
+  const top = sorted.slice(0, maxAgents);
+  const rest = sorted.slice(maxAgents);
+  const othersRuns = rest.reduce((sum, [, count]) => sum + count, 0);
+
+  const result: AgentWorkloadData[] = top.map(([agentId, runCount]) => ({
+    name: truncateName(nameMap.get(agentId) ?? agentId),
+    runs: runCount,
+  }));
+
+  result.push({ name: "Others", runs: othersRuns });
+  return result;
+}
+
+/**
+ * Format a YYYY-MM-DD date string as "MMM dd" (e.g., "Mar 15").
+ * Uses T00:00:00 suffix to avoid timezone shift.
+ */
+export function formatShortDate(dateStr: string): string {
+  const date = new Date(dateStr + "T00:00:00");
+  return date.toLocaleDateString("en-US", { month: "short", day: "numeric" });
+}