feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/presenters/src/lib/pages/issue.page.scss

@@ -192,6 +192,92 @@ a.ts-key {
   }
 }
 
+.ts-inline-form--two {
+  grid-template-columns: minmax(0, 1fr) auto;
+}
+
+// Breakdown: the ticket it hangs from, its checklist and its subtasks.
+.ts-parent {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.375rem;
+  min-width: 0;
+  color: var(--ts-muted);
+  font-size: 0.8125rem;
+  text-decoration: none;
+
+  &:hover {
+    color: var(--ts-brand);
+  }
+}
+
+.ts-parent__title {
+  overflow: hidden;
+  max-width: 18rem;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.ts-checklist,
+.ts-children {
+  display: grid;
+  gap: 0.25rem;
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+.ts-checklist__item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.5rem;
+
+  &.is-done {
+    color: var(--ts-muted);
+    text-decoration: line-through;
+  }
+}
+
+.ts-children {
+  margin-top: 0.75rem;
+}
+
+.ts-child {
+  display: grid;
+  grid-template-columns: auto minmax(0, 1fr) auto auto;
+  gap: 0.625rem;
+  align-items: center;
+  padding: 0.375rem 0;
+  border-top: 1px solid var(--ts-line);
+
+  .ts-comment__avatar {
+    width: 1.5rem;
+    height: 1.5rem;
+    font-size: 0.625rem;
+  }
+}
+
+.ts-child__title {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.ts-parent-form {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+
+  .ts-inline-form {
+    flex: 1;
+  }
+
+  > .ts-muted {
+    margin-top: 0.875rem;
+  }
+}
+
 // Comments: a thread with initials on a rail.
 .ts-comment {
   display: grid;