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>

app.py

@@ -25,6 +25,7 @@ DEFAULT_DATA = {
         "footer_label": "",
         "footer_href": "",
         "profile_image": "",
+        "favicon": "🍯",
         "sections": [],
     }
 }
@@ -121,6 +122,7 @@ def edit_header(page_key):
     page["footer_label"] = request.form.get("footer_label", page.get("footer_label", "")).strip()
     page["footer_href"] = request.form.get("footer_href", page.get("footer_href", "")).strip()
     page["profile_image"] = request.form.get("profile_image", page.get("profile_image", "")).strip()
+    page["favicon"] = request.form.get("favicon", page.get("favicon", "")).strip()
     save_data(data)
     return redirect(url_for("admin_dashboard"))
 
@@ -140,6 +142,7 @@ def add_page():
             "footer_label": "",
             "footer_href": "",
             "profile_image": "",
+            "favicon": "🍯",
             "sections": [],
         }
         save_data(data)