Story 31: View Upcoming Stories

Add GET /workflow/upcoming endpoint that reads .story_kit/stories/upcoming/
and returns story IDs with names parsed from frontmatter. Add UpcomingPanel
component wired into Chat view with loading, error, empty, and list states.

12 new tests (3 backend, 9 frontend) all passing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.test.tsx

@@ -40,6 +40,7 @@ vi.mock("./api/workflow", () => {
 				missing_categories: [],
 			}),
 			getReviewQueueAll: vi.fn().mockResolvedValue({ stories: [] }),
+			getUpcomingStories: vi.fn().mockResolvedValue({ stories: [] }),
 			recordTests: vi.fn(),
 			ensureAcceptance: vi.fn(),
 			getReviewQueue: vi.fn(),