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>

data/links.json

@@ -89,7 +89,8 @@
           }
         ]
       }
-    ]
+    ],
+    "favicon": "🍯"
   },
   "honey": {
     "title": "My Links",
@@ -181,6 +182,7 @@
           }
         ]
       }
-    ]
+    ],
+    "favicon": "🍯"
   }
 }