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/leaderboard.html

@@ -82,7 +82,7 @@
     <main>
         <div id="status">Loading…</div>
         <div id="login-prompt" style="display:none">
-            <p>Sign in to view the leaderboard.</p>
+            <p>Sign in to view the leaderboard. <a href="/account" style="color:var(--accent-hi);text-decoration:none">Create an account</a> if you don't have one.</p>
             <input type="text" id="inp-user" placeholder="Username" autocomplete="username">
             <input type="password" id="inp-pass" placeholder="Password" autocomplete="current-password">
             <div id="error-msg"></div>