feat: add WAU history table, fix retention metric, add new logos and favicon

- Add 12-week Weekly Active Users table to global stats with WoW change %
- Fix 7-day and 30-day retention to measure return on exactly day N (not any day within the window)
- Remove "Avg Guesses Today" stat card
- Update retention description to clarify exact-day measurement
- Add bibdle logos (SVG, square PNG, circle PNG) and new favicon.png
- Wire favicon.png as the site favicon via app.html link tag

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

src/app.html

@@ -4,6 +4,7 @@
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
 		<script src="https://rybbit.snail.city/api/script.js" data-site-id="9abf0e81d024" defer></script>
+		<link rel="icon" href="/favicon.png" type="image/png" />
 		%sveltekit.head%
 	</head>
 	<body data-sveltekit-preload-data="hover">