feat(18-02): create PhaseAccordion component

Expandable/collapsible phase container with chevron toggle, phase
number + name header, task count (completed/total), StatusBadge,
phase-level DependencyIndicator, and TaskRow list when expanded.

packages/web/src/components/PhaseAccordion.tsx

@@ -0,0 +1,102 @@
+import { useState } from "react";
+import { ChevronDown, ChevronRight } from "lucide-react";
+import { StatusBadge } from "@/components/StatusBadge";
+import { DependencyIndicator } from "@/components/DependencyIndicator";
+import { TaskRow, type SerializedTask } from "@/components/TaskRow";
+
+/** Phase shape as returned by tRPC (Date fields serialized to string over JSON) */
+interface SerializedPhase {
+  id: string;
+  initiativeId: string;
+  number: number;
+  name: string;
+  description: string | null;
+  status: string;
+  createdAt: string;
+  updatedAt: string;
+}
+
+/** Task entry with associated metadata, pre-assembled by the parent page */
+interface TaskEntry {
+  task: SerializedTask;
+  agentName: string | null;
+  blockedBy: Array<{ name: string; status: string }>;
+}
+
+interface PhaseAccordionProps {
+  phase: SerializedPhase;
+  tasks: TaskEntry[];
+  defaultExpanded: boolean;
+  phaseDependencies: Array<{ name: string; status: string }>;
+  onTaskClick: (taskId: string) => void;
+}
+
+export function PhaseAccordion({
+  phase,
+  tasks,
+  defaultExpanded,
+  phaseDependencies,
+  onTaskClick,
+}: PhaseAccordionProps) {
+  const [expanded, setExpanded] = useState(defaultExpanded);
+
+  const completedCount = tasks.filter(
+    (t) => t.task.status === "completed",
+  ).length;
+  const totalCount = tasks.length;
+
+  return (
+    <div className="border-b border-border">
+      {/* Phase header — clickable to toggle */}
+      <button
+        className="flex w-full items-center gap-3 px-4 py-3 text-left hover:bg-accent/50"
+        onClick={() => setExpanded((prev) => !prev)}
+      >
+        {/* Expand / collapse chevron */}
+        {expanded ? (
+          <ChevronDown className="h-4 w-4 shrink-0 text-muted-foreground" />
+        ) : (
+          <ChevronRight className="h-4 w-4 shrink-0 text-muted-foreground" />
+        )}
+
+        {/* Phase number + name */}
+        <span className="min-w-0 flex-1 truncate font-medium">
+          Phase {phase.number}: {phase.name}
+        </span>
+
+        {/* Task count */}
+        <span className="shrink-0 text-sm text-muted-foreground">
+          ({completedCount}/{totalCount})
+        </span>
+
+        {/* Phase status */}
+        <StatusBadge status={phase.status} className="shrink-0" />
+      </button>
+
+      {/* Phase-level dependency indicator (when blocked by another phase) */}
+      {phaseDependencies.length > 0 && (
+        <DependencyIndicator
+          blockedBy={phaseDependencies}
+          type="phase"
+          className="pb-2 pl-11"
+        />
+      )}
+
+      {/* Expanded task list */}
+      {expanded && (
+        <div className="pb-3 pl-10 pr-4">
+          {tasks.map((entry, idx) => (
+            <TaskRow
+              key={entry.task.id}
+              task={entry.task}
+              agentName={entry.agentName}
+              blockedBy={entry.blockedBy}
+              isLast={idx === tasks.length - 1}
+              onClick={() => onTaskClick(entry.task.id)}
+            />
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}