feat(UI-Overhaul-Phase7): Reports UI mit ResizablePanel und PluginToolbar

Migration 0139: Add folder_id column to report_templates for folder-based organization

Backend:
- ReportTemplate model: add folder_id field (nullable UUID)

Frontend:
- api/reports.ts: ReportTemplate interface updated with folder_id
- Reports.tsx: Added ResizablePanel for template list (resizable left sidebar)
- Reports.tsx: Added PluginToolbar registration with new-report button
- Reports.tsx: Added useEffect import for toolbar registration

tsc clean

app/plugins/builtins/report_generator/models.py

@@ -34,6 +34,7 @@ class ReportTemplate(Base, TenantMixin, OwnedMixin):
         String(50), nullable=False, default="csv"
     )
     created_by: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False)
+    folder_id: Mapped[uuid.UUID | None] = mapped_column(PGUUID(as_uuid=True), nullable=True)
 
 
 class ReportInstance(Base, TenantMixin, OwnedMixin):