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/lib/auth-middleware.test.ts

@@ -36,7 +36,7 @@ import {
   loadAuthFromCookies,
 } from "./pocketbase";
 
-const mockLogger = logger as {
+const mockLogger = logger as unknown as {
   warn: ReturnType<typeof vi.fn>;
   error: ReturnType<typeof vi.fn>;
   info: ReturnType<typeof vi.fn>;