Add favicon and make profile picture editable per page

- Favicon: emoji-based inline SVG (🍯), the old /images/favicon/favicon.ico
  reference never resolved to an actual file.
- Profile picture (the circular image at the top) is now a per-page
  "profile_image" field editable in /admin, instead of a hardcoded
  URL in the template.

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

app.py

@@ -24,6 +24,7 @@ DEFAULT_DATA = {
         "subtitle2": "",
         "footer_label": "",
         "footer_href": "",
+        "profile_image": "",
         "sections": [],
     }
 }
@@ -119,6 +120,7 @@ def edit_header(page_key):
     page["subtitle2"] = request.form.get("subtitle2", page.get("subtitle2", "")).strip()
     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()
     save_data(data)
     return redirect(url_for("admin_dashboard"))
 
@@ -137,6 +139,7 @@ def add_page():
             "subtitle2": "",
             "footer_label": "",
             "footer_href": "",
+            "profile_image": "",
             "sections": [],
         }
         save_data(data)