[Enhancement]: Consider adding a dashboard layout with left sidebar navigation

### Is your feature request related to a problem?

SimBoard currently uses a top navigation bar, which may not scale well as the application adds more workflows such as browsing, comparison, uploads, diagnostics, resources, AI features, and admin/ingestion views.

### Describe the solution you'd like

Update the main application shell to use a dashboard-style layout with:

- A left sidebar for primary navigation
- A minimal top bar for global context and actions
- Clear active-page state
- Responsive behavior for smaller screens

Suggested sidebar items:

- Browse
- Compare
- Submit Simulation
- Diagnostics
- Resources
- Ingestion / Admin, if applicable

### Describe alternatives you've considered

Keep the existing top navbar, but this may become crowded as SimBoard grows.

### Additional context

The goal is to make SimBoard feel more like a scalable scientific operations application while keeping the UI simple and not overly enterprise-heavy.