feat: Add Pro features discovery banner

Adds a dismissible banner in the topbar that informs users about
PLANKA Pro features.

- Dismissible per-user (stored in localStorage)
- Reappears after 30 days
- Rotates between main message and 3 feature highlights
- Links to planka.app/pro (with ref parameter for anonymous source attribution)

client/src/components/users/UserActionsStep/UserActionsStep.jsx

@@ -1,5 +1,5 @@
 /*!
- * Copyright (c) 2024 PLANKA Software GmbH
+ * Copyright (c) 2026 PLANKA Software GmbH
  * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
  */
 
@@ -104,6 +104,17 @@ const UserActionsStep = React.memo(({ onClose }) => {
               context: 'title',
             })}
           </Menu.Item>
+          <Menu.Item
+            href="https://planka.app/pro?ref=app-menu"
+            target="_blank"
+            rel="noopener noreferrer"
+            className={styles.proMenuItem}
+          >
+            <Icon name="gem" className={styles.proMenuItemIcon} />
+            {withAdministration
+              ? t('common.upgradeTeamToPro', { context: 'title' })
+              : t('common.discoverPlankaPro', { context: 'title' })}
+          </Menu.Item>
           <hr className={styles.divider} />
           <Menu.Item
             {...logoutMenuItemProps} // eslint-disable-line react/jsx-props-no-spreading

client/src/components/users/UserActionsStep/UserActionsStep.module.scss

@@ -1,5 +1,5 @@
 /*!
- * Copyright (c) 2024 PLANKA Software GmbH
+ * Copyright (c) 2026 PLANKA Software GmbH
  * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
  */
 
@@ -25,4 +25,14 @@
     float: left;
     margin: 0 0.5em 0 0;
   }
+
+  .proMenuItem {
+    margin: 0;
+    padding-left: 14px;
+  }
+
+  .proMenuItemIcon {
+    float: left;
+    margin: 0 0.5em 0 0;
+  }
 }