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/api/workflow.test.ts

@@ -98,6 +98,28 @@ describe("workflowApi", () => {
 		});
 	});
 
+	describe("getUpcomingStories", () => {
+		it("sends GET to /workflow/upcoming", async () => {
+			const response = {
+				stories: [
+					{ story_id: "31_view_upcoming", name: "View Upcoming" },
+					{ story_id: "32_worktree", name: null },
+				],
+			};
+			mockFetch.mockResolvedValueOnce(okResponse(response));
+
+			const result = await workflowApi.getUpcomingStories();
+
+			expect(mockFetch).toHaveBeenCalledWith(
+				"/api/workflow/upcoming",
+				expect.objectContaining({}),
+			);
+			expect(result.stories).toHaveLength(2);
+			expect(result.stories[0].name).toBe("View Upcoming");
+			expect(result.stories[1].name).toBeNull();
+		});
+	});
+
 	describe("getReviewQueue", () => {
 		it("sends GET to /workflow/review", async () => {
 			mockFetch.mockResolvedValueOnce(