## Why

The app currently has no navigation — just a single page at `/`. Adding a sidebar drawer with navigation enables the app to grow to multiple pages while providing standard MudBlazor layout structure (AppBar + Drawer + MainContent).

## What Changes

- Add a MudDrawer to MainLayout with a NavMenu component containing a "Sales Assistant" link
- Add a hamburger toggle button in the AppBar to open/close the drawer
- Move the existing chat page from `/` to `/sales-assistant`
- Add a landing page or redirect at `/` so the app has a default route

## Capabilities

### New Capabilities
- `sidebar-navigation`: Collapsible sidebar drawer with navigation menu, hamburger toggle, and route structure

### Modified Capabilities
- `chat-ui`: Route changes from `/` to `/sales-assistant`

## Impact

- **MainLayout.razor**: Add MudDrawer, hamburger icon, drawer toggle state
- **New NavMenu component**: Shared/NavMenu.razor with MudNavGroup/MudNavLink items
- **Chat.razor**: Route changes from `@page "/"` to `@page "/sales-assistant"`
- **Chat.razor.css**: Height calc may need adjustment if AppBar Dense changes
- No new packages — MudDrawer/MudNavMenu are part of MudBlazor