feat: add scoreboard with player names, per-category leaderboards, and themed tables

Add player_name column to games (migration included), with regex-based
validation and anti-abuse sanitization. New /scoreboard page shows recent
games from localStorage, popular categories with play counts, and top-10
per-category leaderboards sorted by completion time. Also includes
two-click reverse word matching, base URL prefix support for reverse-proxy
hosting, BasePageHandler refactoring, themed table CSS for all 5 themes,
and comprehensive test coverage for player names and scoreboard API.

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

templates/main.html

@@ -12,6 +12,6 @@
         Find hidden words in a grid of letters. Words can appear horizontally,
         vertically, or diagonally. How fast can you find them all?
     </p>
-    <a href="/game" class="btn btn-accent btn-lg px-5">🎮 Start Game</a>
+    <a href="{{ base_url }}game" class="btn btn-accent btn-lg px-5">🎮 Start Game</a>
 </header>
 {% end %}