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/src/lib.rs

@@ -213,6 +213,10 @@ fn build_router_inner(state: AppState, rate_limit: bool) -> Router {
             "/play",
             get(|| async { Html(include_str!("../web/game.html")) }),
         )
+        .route(
+            "/account",
+            get(|| async { Html(include_str!("../web/account.html")) }),
+        )
         .route(
             "/leaderboard",
             get(|| async { Html(include_str!("../web/leaderboard.html")) }),