feat(FN-1674): add roadmap export and handoff system

- Add RoadmapStore with read APIs for project-scoped roadmap data
- Add roadmap-handoff mapper to transform roadmap data for export
- Add project-scoped handoff API route (/api/projects/:id/roadmap/handoff)
- Add useRoadmaps hook for fetching and exposing roadmap data to components
- Update RoadmapsView with export/handoff UX path and roadmap detail view
- Add roadmap routes with project-scoped handoff endpoint
- Add comprehensive tests for handoff mapper and roadmap routes
- Update architecture.md and add dashboard-guide.md documentation

packages/dashboard/app/components/__tests__/RoadmapsView.test.tsx

@@ -41,6 +41,9 @@ vi.mock("lucide-react", () => ({
   X: (props: unknown) => <span data-testid="x-icon" {...props}>X</span>,
   GripVertical: (props: unknown) => <span data-testid="grip-icon" {...props}>Grip</span>,
   Sparkles: (props: unknown) => <span data-testid="sparkles-icon" {...props}>Sparkles</span>,
+  Download: (props: unknown) => <span data-testid="download-icon" {...props}>Download</span>,
+  Copy: (props: unknown) => <span data-testid="copy-icon" {...props}>Copy</span>,
+  Loader: (props: unknown) => <span data-testid="loader-icon" {...props}>Loader</span>,
 }));
 
 const mockRoadmaps: Roadmap[] = [