feat: navigation restructure + drag-and-drop menu ordering

Navigation:
- Remove Plugins header from sidebar
- Merge static items and plugin items into unified sorted list
- Group related menu items (Dateien, E-Mail, Kalender, Automation)
- German labels for all menu items
- Sort by order field, alphabetical fallback for ties

Drag-and-Drop Menu:
- New backend endpoints: GET/PUT /api/v1/users/me/menu-order
- Store menu order in user preferences JSONB field
- New SettingsMenuOrder page with @dnd-kit drag-and-drop
- New Settings tab: Menu ordering
- Sidebar reads saved menu order and sorts accordingly
- Reset to default option

app/plugins/builtins/mail/plugin.py

@@ -57,8 +57,8 @@ class MailPlugin(BasePlugin):
         migrations=["0001_initial.sql", "0006_flag_type.sql", "0007_sync_queue.sql", "0008_sync_queue_deleted_at.sql", "0009_remove_mail_soft_delete.sql", "0010_add_deleted_at.sql"],
         permissions=["mail:read", "mail:send", "mail:config", "mail:share", "mail:write", "mail:delete"],
         menu_items=[
-            FrontendMenuItem(label_key='nav.email', label='Mail', path='/mail', icon='Mail', order=30),
-            FrontendMenuItem(label_key='nav.emailSettings', label='Mail Settings', path='/mail/settings', icon='Settings', group='nav.email', order=31),
+            FrontendMenuItem(label_key='nav.mail', label='E-Mail', path='/mail', icon='Mail', group='E-Mail', order=30),
+            FrontendMenuItem(label_key='nav.mail.settings', label='Einstellungen', path='/mail/settings', icon='Settings', group='E-Mail', order=31),
         ],
         page_routes=[
             FrontendPageRoute(path='/mail', component='@/pages/Mail', protected=True),