feat(web): account page with sign in / sign up tabs

- Add account.html: tabbed form for login and registration, signed-in
  state with sign-out, links to leaderboard and replays
- Wire /account route in build_router_inner
- Add Account card to landing page
- Link leaderboard login prompt to /account for new users

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

solitaire_server/web/home.html

@@ -148,6 +148,14 @@
                 <div class="card-desc">Watch recent completed games</div>
             </div>
         </a>
+
+        <a class="card" href="/account">
+            <div class="card-icon">&#9786;</div>
+            <div class="card-body">
+                <div class="card-title">Account</div>
+                <div class="card-desc">Sign in or create a new account</div>
+            </div>
+        </a>
     </nav>
 
     <footer>v0.1.0</footer>