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/ai_assistant/plugin.py

@@ -41,7 +41,7 @@ class AIAssistantPlugin(BasePlugin):
         ],
         is_core=True,
         menu_items=[
-            FrontendMenuItem(label_key='nav.aiAssistant', label='AI Assistant', path='/ai-assistant', icon='Bot', order=90),
+            FrontendMenuItem(label_key='nav.aiAssistant', label='KI Assistent', path='/ai-assistant', icon='Bot', order=90),
         ],
         page_routes=[
             FrontendPageRoute(path='/ai-assistant', component='@/pages/AIAssistant', protected=True),

app/plugins/builtins/automation/plugin.py

@@ -75,13 +75,15 @@ class AutomationPlugin(BasePlugin):
                 label="Automation",
                 path="/automation",
                 icon="Zap",
+                group="Automation",
                 order=50,
             ),
             FrontendMenuItem(
                 label_key="nav.agents",
-                label="Agents",
+                label="Agenten",
                 path="/agents",
                 icon="Bot",
+                group="Automation",
                 order=51,
             ),
         ],

app/plugins/builtins/calendar/plugin.py

@@ -42,8 +42,8 @@ class CalendarPlugin(BasePlugin):
             "calendar:admin",
         ],
         menu_items=[
-            FrontendMenuItem(label_key='nav.calendar', label='Calendar', path='/calendar', icon='Calendar', order=20),
-            FrontendMenuItem(label_key='nav.calendarKanban', label='Calendar Kanban', path='/calendar/kanban', icon='KanbanSquare', group='nav.calendar', order=21),
+            FrontendMenuItem(label_key='nav.calendar', label='Kalender', path='/calendar', icon='Calendar', group='Kalender', order=20),
+            FrontendMenuItem(label_key='nav.calendar.kanban', label='Kanban', path='/calendar/kanban', icon='KanbanSquare', group='Kalender', order=21),
         ],
         page_routes=[
             FrontendPageRoute(path='/calendar', component='@/pages/Calendar', protected=True),

app/plugins/builtins/dms/plugin.py

@@ -32,8 +32,8 @@ class DmsPlugin(BasePlugin):
             "dms:admin",
         ],
         menu_items=[
-            FrontendMenuItem(label_key='nav.files', label='Files', path='/dms', icon='FolderOpen', order=40),
-            FrontendMenuItem(label_key='nav.trash', label='Trash', path='/dms/trash', icon='Trash2', group='nav.files', order=41),
+            FrontendMenuItem(label_key='nav.dms', label='Dateien', path='/dms', icon='FolderOpen', group='Dateien', order=40),
+            FrontendMenuItem(label_key='nav.dms.trash', label='Papierkorb', path='/dms/trash', icon='Trash2', group='Dateien', order=41),
         ],
         page_routes=[
             FrontendPageRoute(path='/dms', component='@/pages/Dms', protected=True),

app/plugins/builtins/kommunikation/plugin.py

@@ -50,7 +50,7 @@ class KommunikationPlugin(BasePlugin):
         ],
         is_core=True,
         menu_items=[
-            FrontendMenuItem(label_key='nav.communication', label='Communication', path='/communication', icon='MessageSquare', order=80),
+            FrontendMenuItem(label_key='nav.communication', label='Kommunikation', path='/communication', icon='MessageSquare', order=80),
         ],
         page_routes=[
             FrontendPageRoute(path='/communication', component='@/pages/Communication', protected=True),

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),

app/plugins/builtins/report_generator/plugin.py

@@ -27,7 +27,7 @@ class ReportGeneratorPlugin(BasePlugin):
         migrations=["0001_initial.sql"],
         permissions=["reports:read", "reports:generate", "reports:manage_templates"],
         menu_items=[
-            FrontendMenuItem(label_key='nav.reports', label='Reports', path='/reports', icon='BarChart3', order=70),
+            FrontendMenuItem(label_key='nav.reports', label='Berichte', path='/reports', icon='BarChart3', order=70),
         ],
         page_routes=[
             FrontendPageRoute(path='/reports', component='@/pages/Reports', protected=True),

app/plugins/builtins/tasks/plugin.py

@@ -39,7 +39,7 @@ class TasksPlugin(BasePlugin):
         menu_items=[
             FrontendMenuItem(
                 label_key="nav.tasks",
-                label="Tasks",
+                label="Aufgaben",
                 path="/tasks",
                 icon="CheckSquare",
                 order=30,