Make favicon editable per page in /admin

Same emoji-based inline SVG favicon as before, but the emoji is now
a per-page "favicon" field editable in the header form instead of
hardcoded in the template.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/page.html

@@ -4,7 +4,7 @@
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
   <title>Honey | Links</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>">
+  <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>{{ page.favicon or '🍯' }}</text></svg>">
   <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600&display=swap" rel="stylesheet"/>
   <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
 </head>