UI: unified design system, light/dark themes, redesigned pages and player UI

Tokens and components in styles.css, shared Ui (dialogs, toasts), icons,
bottom tab bar on mobile, keyboard/gamepad navigation, library shelf with
filters and conversion badges, hero game page, restyled player screens.
Games list now returns per-game version counts. Adds docs/UI_DESIGN.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/index.html

@@ -2,8 +2,16 @@
 <html lang="ru">
   <head>
     <meta charset="UTF-8" />
-    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
+    <meta name="theme-color" content="#0e1016" />
     <title>PGames</title>
+    <script>
+      // apply the saved theme before first paint to avoid a flash
+      try {
+        var t = localStorage.getItem("pg-theme");
+        if (t === "light" || t === "dark") document.documentElement.dataset.theme = t;
+      } catch (e) {}
+    </script>
   </head>
   <body>
     <div id="root"></div>