Prevent a super admin from changing their own access level

editUser() let a super user submit any type for any target, including
themselves - accidentally downgrading your own account could lock you
out of admin functions. Self-edits now keep the existing type
regardless of what was submitted; the type radios are disabled in the
UI for that case with an explanatory note.

src/forms/form_users.php

@@ -26,25 +26,29 @@
     </div>
 
     <?php if ($_SESSION['type'] === 'super'): ?>
+    <?php $editing_self = $edit && (int) $user['id'] === (int) $_SESSION['user_id']; ?>
     <div class="col-sm-4">
         <label for="user-type">User type *</label>
 
         <div class="form-group">
             <div class="radio">
                 <label class="radio">
-                <input type="radio" name="type" value="super" required="required" <?php echo ($edit && $user['type'] =='super') ? "checked": "" ; ?>/> Super admin</label>
+                <input type="radio" name="type" value="super" required="required" <?php echo ($edit && $user['type'] =='super') ? "checked": "" ; ?> <?php echo $editing_self ? "disabled" : ""; ?>/> Super admin</label>
             </div>
 
             <div class="radio">
                 <label class="radio">
-                <input type="radio" name="type" value="admin" required="required" <?php echo ($edit && $user['type'] =='admin') ? "checked": "" ; ?>/> Admin</label>
+                <input type="radio" name="type" value="admin" required="required" <?php echo ($edit && $user['type'] =='admin') ? "checked": "" ; ?> <?php echo $editing_self ? "disabled" : ""; ?>/> Admin</label>
             </div>
 
             <div class="radio">
                 <label class="radio">
-                <input type="radio" name="type" value="user" required="required" id="type-user" <?php echo ($edit && $user['type'] =='user') ? "checked": "" ; ?>/> User (read-only)</label>
+                <input type="radio" name="type" value="user" required="required" id="type-user" <?php echo ($edit && $user['type'] =='user') ? "checked": "" ; ?> <?php echo $editing_self ? "disabled" : ""; ?>/> User (read-only)</label>
             </div>
         </div>
+        <?php if ($editing_self): ?>
+        <small class="form-text text-muted">You can't change your own access level.</small>
+        <?php endif; ?>
     </div>
 
     <div class="col-sm-12 mt-2" id="user-view-toggles">

src/lib/Users/Users.php

@@ -160,7 +160,14 @@ class Users
             'edit' => "true",
         ));
 
-        $requested_type = $_SESSION['type'] === 'admin' ? 'user' : ($input_data['type'] ?? '');
+        $is_self_edit = (int) $input_data['id'] === (int) $_SESSION['user_id'];
+
+        // A user editing their own account keeps their current type, even if a
+        // different value was submitted - prevents accidentally (or deliberately)
+        // locking yourself out by downgrading your own access level.
+        $requested_type = $_SESSION['type'] === 'admin'
+            ? 'user'
+            : ($is_self_edit ? $target['type'] : ($input_data['type'] ?? ''));
 
         $validation_error = $this->validateUsernameAndType($input_data['username'] ?? '', $requested_type);
         if ($validation_error !== null) {