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/admin.html

@@ -141,6 +141,10 @@
         <label class="checkbox">Profilbild-URL</label>
         <input type="text" name="profile_image" value="{{ page.profile_image or '' }}" placeholder="https://...">
       </div>
+      <div>
+        <label class="checkbox">Favicon (Emoji)</label>
+        <input type="text" name="favicon" value="{{ page.favicon or '' }}" placeholder="🍯" style="width: 60px;">
+      </div>
       <button type="submit">Header speichern</button>
     </form>