fix(inspector): prevent error flickering (#3978)

Closes FRONT-915

### TL;DR

Improved UI components and error handling for the Rivet application, with focus on sidebar layout, actor error handling, and query caching.

### What changed?

- **Sidebar Layout**: Restructured the sidebar navigation with proper sizing and layout for buttons, wrapped links in a flex container, and added proper icon placement
- **Actor Error Handling**: Enhanced error handling in `ActorStatusLabel` and `RunnerPoolError` components with better type definitions and pattern matching
- **Create Actor Dialog**: Fixed datacenter parameter to use `undefined` instead of empty string for inspector app type
- **Query Caching**: Implemented a `previousQueryCache` to prevent redundant query invalidation and improve performance
- **Loading Component**: Replaced `PendingRouteLayout` with `FullscreenLoading` for namespace routes

### How to test?

1. Verify the sidebar layout and buttons display correctly with proper sizing
2. Test actor error handling by triggering various error conditions
3. Create an actor in inspector mode and verify datacenter parameter is handled correctly
4. Navigate between pages to ensure query caching works properly without redundant invalidations
5. Check that namespace routes show the correct loading component

### Why make this change?

These changes improve the user experience by:

- Creating a more consistent and visually appealing sidebar layout
- Providing better error handling and more descriptive error messages for actor operations
- Optimizing query performance by preventing unnecessary invalidations
- Using appropriate loading components for a smoother user experience during navigation

The code structure is now more maintainable with better type definitions and pattern matching for error handling.

Author: jog1t