Add character rename to admin panel

Renaming checks for a name collision first (characters.char_name has
no unique constraint in the schema, so this has to be enforced at the
application level to avoid two characters silently sharing a name),
and also updates list_friend.namefriend for any rows referencing this
character's id -- that column is a denormalized display-name cache in
other players' friend lists and would otherwise go stale on rename.

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

webadmin/templates/edit_character.html

@@ -4,6 +4,9 @@
 <div class="card" style="max-width:600px;">
   <h2 style="margin-top:0;">Charakter: {{ char.char_name }} <span class="badge ok">Account: {{ char.account_name }}</span></h2>
   <form method="post">
+    <label>Charaktername</label>
+    <input type="text" name="char_name" value="{{ char.char_name }}" required>
+
     <label>Standort (wirkt beim nächsten Login)</label>
     <select name="respawnid">
       <option value="{{ char.respawnid }}">-- aktuell: Zone #{{ char.respawnid }} --</option>