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/login/page.tsx

@@ -97,17 +97,23 @@ export default function LoginPage() {
   // Show loading state while checking auth methods
   if (isCheckingAuth) {
     return (
-      <div className="flex min-h-screen items-center justify-center">
+      <main
+        id="main-content"
+        className="flex min-h-screen items-center justify-center"
+      >
         <div className="w-full max-w-md space-y-8 p-8">
           <h1 className="text-2xl font-bold text-center">PhaseFlow</h1>
           <div className="text-center text-gray-500">Loading...</div>
         </div>
-      </div>
+      </main>
     );
   }
 
   return (
-    <div className="flex min-h-screen items-center justify-center">
+    <main
+      id="main-content"
+      className="flex min-h-screen items-center justify-center"
+    >
       <div className="w-full max-w-md space-y-8 p-8">
         <h1 className="text-2xl font-bold text-center">PhaseFlow</h1>
 
@@ -181,6 +187,6 @@ export default function LoginPage() {
           </form>
         )}
       </div>
-    </div>
+    </main>
   );
 }