feat: due dates, subtasks, and task detail page (HQ-{number} URLs)

- Schema: added due_date and subtasks JSONB columns to tasks
- API: CRUD endpoints for subtasks (/tasks/:id/subtasks)
- API: due date support in create/update task
- TaskDetailPanel: due date picker with overdue/soon badges
- TaskDetailPanel: subtask checklist with progress bar
- TaskPage: full-page task view at /task/HQ-{number}
- Dashboard: task cards link to detail page, show subtask progress & due date badges
- Migration: 0001_mighty_callisto.sql

frontend/src/lib/api.ts

@@ -115,6 +115,38 @@ export async function deleteProject(id: string): Promise<void> {
   if (!res.ok) throw new Error("Failed to delete project");
 }
 
+// Subtasks
+export async function addSubtask(taskId: string, title: string): Promise<Task> {
+  const res = await fetch(`${BASE}/${taskId}/subtasks`, {
+    method: "POST",
+    credentials: "include",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ title }),
+  });
+  if (!res.ok) throw new Error("Failed to add subtask");
+  return res.json();
+}
+
+export async function toggleSubtask(taskId: string, subtaskId: string, completed: boolean): Promise<Task> {
+  const res = await fetch(`${BASE}/${taskId}/subtasks/${subtaskId}`, {
+    method: "PATCH",
+    credentials: "include",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ completed }),
+  });
+  if (!res.ok) throw new Error("Failed to toggle subtask");
+  return res.json();
+}
+
+export async function deleteSubtask(taskId: string, subtaskId: string): Promise<Task> {
+  const res = await fetch(`${BASE}/${taskId}/subtasks/${subtaskId}`, {
+    method: "DELETE",
+    credentials: "include",
+  });
+  if (!res.ok) throw new Error("Failed to delete subtask");
+  return res.json();
+}
+
 // Progress Notes
 export async function addProgressNote(taskId: string, note: string): Promise<Task> {
   const res = await fetch(`${BASE}/${taskId}/notes`, {