Add accessibility improvements (P4.2 partial)

- Add skip navigation link to root layout
- Add semantic HTML landmarks (main element) to login and settings pages
- Add aria-labels to calendar day buttons with date, cycle day, and phase info
- Add id="main-content" to dashboard main element for skip link target
- Fix pre-existing type error in auth-middleware.test.ts

Tests: 781 passing (11 new accessibility tests)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/settings/page.test.tsx

@@ -505,4 +505,24 @@ describe("SettingsPage", () => {
       expect(screen.queryByText(/settings saved/i)).not.toBeInTheDocument();
     });
   });
+
+  describe("accessibility", () => {
+    it("wraps content in a main element", async () => {
+      render(<SettingsPage />);
+
+      await waitFor(() => {
+        expect(screen.getByRole("main")).toBeInTheDocument();
+      });
+    });
+
+    it("has proper heading structure with h1", async () => {
+      render(<SettingsPage />);
+
+      await waitFor(() => {
+        const heading = screen.getByRole("heading", { level: 1 });
+        expect(heading).toBeInTheDocument();
+        expect(heading).toHaveTextContent(/settings/i);
+      });
+    });
+  });
 });