Native Sub-Menus / Nested Navigation Groups in App Layout

**Summary**
Users need the ability to group navigation items into **Sub-Menus** (Folders) within the main App Layout. Currently, the navigation sidebar/tab bar is a flat list. To organize complex apps, developers are forced to create "Menu Pages" (screens containing only buttons or collections linking to other screens), which adds unnecessary friction and clicks for the end user.

**Current Problem**
* **The "Menu Screen" Workaround:** Because there is no native hierarchy, developers have to create a top-level screen called "Settings" or "Admin", which is just a blank page with buttons leading to "Users", "Billing", and "Logs". This forces the user to load a page just to navigate to another page (2 clicks instead of 1).
* **Visual Clutter:** As an app grows, the sidebar becomes an endless flat list of 15+ items. This is overwhelming for users and bad for UI design.
* **Limited Space:** On mobile devices (Tab Bar), there is strictly limited space (usually 4-5 items). Without sub-menus (or a "More" drawer), complex navigation is impossible to implement cleanly.

**Examples/Scenarios**
* **Settings Section:** Instead of cluttering the main menu with *Profile, Security, Notifications, Billing*, these should be nested under a single **"Settings"** parent item.
* **Admin Portals:** A manager needs access to "Employees", "Contractors", and "Payroll". These should be grouped under **"HR Management"**.
* **Departmental Grouping:** An app serving multiple departments needs to group screens by "Sales", "Operations", and "Support" to keep the interface clean.

**Why This Matters**
* **Scalability:** Allows apps to grow without the navigation interface breaking.
* **Standard UX Pattern:** Accordion menus or Dropdown menus are standard in almost all web applications (SaaS, Admin Panels).
* **Better Navigation:** Users can find what they need faster without loading intermediate "hub" pages.

**Suggested UX**
* **Navigation Builder:** In the Layout Editor, allow adding a **"Folder"** or **"Group"** item.
* **Drag & Drop:** Allow dragging existing screens *inside* this Group.
* **Frontend Behavior:**
* **Desktop (Sidebar):** Clicking the Group expands/collapses the list (Accordion style).
* **Mobile (Tabs/Drawer):** Clicking the Group opens a secondary selection sheet or expands the list.