Add favicon; fix two server bugs found deploying to socoban.08h.ru

- Favicon: 🚀 emoji via inline SVG data URI, both sokoban.html and php/index.php
- api.php: mb_substr() -> substr() (mbstring not enabled on the server)
- api.php: MariaDB rejects LIMIT directly inside NOT IN(...) (error 1235) —
  wrap the top-5 subquery in an extra derived-table SELECT
Verified live against the deployed MySQL/MariaDB DB: whoami, set_name,
submit (insert + dedup + trim-to-5), top all round-trip correctly.

php/index.php

@@ -4,6 +4,7 @@
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>Sokoban — 60 Levels</title>
+<link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>🚀</text></svg>">
 <style>
   * { margin: 0; padding: 0; box-sizing: border-box; }
   .hidden { display: none !important; }