feat(sidebar): inline label create, live sync, and kebab submenu parity (#706)

Portal-wrap DropdownMenuSubContent so the kebab Labels submenu renders outside
the clipped dropdown container, matching ContextMenuSubContent and fixing the
empty/broken kebab submenu.

Thread onLabelsChanged from LabelsSection through SettingsModal to
EditorLayout so label creates and deletes in Settings propagate to the
sidebar menus without a page refresh.

Add an inline "New label" form in both the kebab and context menu label
submenus that creates and assigns a label in one interaction, removing the
Settings round-trip from the label assignment flow.

frontend/src/hooks/__tests__/useStackMenuItems.test.tsx

@@ -27,6 +27,7 @@ function makeCtx(overrides: Partial<StackMenuCtx> = {}): StackMenuCtx {
     pin: vi.fn(),
     unpin: vi.fn(),
     toggleLabel: vi.fn(),
+    createAndAssignLabel: vi.fn(),
     openLabelManager: vi.fn(),
     ...overrides,
   };