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>

handlers/main.py

@@ -0,0 +1,10 @@
+"""Landing page handler."""
+
+from handlers import BasePageHandler
+
+
+class MainHandler(BasePageHandler):
+    """Serves the home page."""
+
+    def get(self):
+        self.render("main.html")