feat(plan): GanttPlanView top-level component

Composes one ProjectSection per section with a single shared
timeline range. Renders the 'Nothing scheduled yet' empty
state when no section has any dated cards or subtasks.

src/components/roadmap/gantt-plan-view.tsx

@@ -0,0 +1,56 @@
+"use client";
+
+import { useMemo } from "react";
+
+import { ProjectSection } from "@/components/roadmap/project-section";
+import { timelineRange } from "@/lib/roadmap/timeline";
+import type { PlanSection } from "@/lib/roadmap/load";
+
+/**
+ * Top-level Gantt view. Computes one shared timeline range across every
+ * project section so a card in one project and a card in another share
+ * the same month/week axis. When every card and every subtask has no
+ * dates, shows the "Nothing scheduled yet" empty state.
+ */
+export function GanttPlanView({ sections }: { sections: PlanSection[] }) {
+  const range = useMemo(() => {
+    let n = 0;
+    const items: { id: string; startDate: string; targetDate: string }[] = [];
+    for (const s of sections) {
+      for (const c of s.cards) {
+        if (c.startDate || c.targetDate) {
+          items.push({ id: `c-${++n}`, startDate: c.startDate, targetDate: c.targetDate });
+        }
+        for (const st of c.datedSubtasks) {
+          items.push({ id: `s-${++n}`, startDate: st.startDate, targetDate: st.targetDate });
+        }
+      }
+    }
+    return timelineRange(items);
+  }, [sections]);
+
+  if (sections.length === 0) {
+    return (
+      <div className="rounded-[var(--radius-tile)] border-2 border-dashed border-[var(--border-subtle)] px-4 py-12 text-center text-[var(--text-muted)]">
+        <h2 className="text-base font-medium text-[var(--text-primary)]">
+          Nothing scheduled yet
+        </h2>
+        <p className="mx-auto mt-1.5 max-w-sm text-sm">
+          Set a start date and target date on a card to see it appear here.
+        </p>
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-4">
+      {sections.map((section) => (
+        <ProjectSection
+          key={section.project.id}
+          section={section}
+          range={range}
+        />
+      ))}
+    </div>
+  );
+}