Show GPU renderer FPS in PerformanceHUD when GPU is active

[CREVIOS]

Problem

The PerformanceHUD shows FPS from the frontend (frames received via IPC). When the GPU renderer is active, the real FPS is tracked in the Rust `GpuRenderer` but not exposed to the UI.

Implementation

Step 1: Expose GPU metrics via Tauri command

Add a command `get_gpu_metrics()` that returns:

```typescript
interface GpuMetrics {
fps: number;
frameTime: number; // ms per frame
publishTime: number; // ms for SharedFrame publish
uploadTime: number; // ms for write_texture
presentTime: number; // ms for submit + present
framesRendered: number;
textureWidth: number;
textureHeight: number;
adapterName: string;
}
```

Step 2: Store metrics in GpuRenderer

The FPS counter already exists (`frames_rendered`, `last_fps_log`). Add a `Mutex` field that's updated every frame and readable from any thread.

Step 3: Display in PerformanceHUD

When GPU renderer is active:

• Show "GPU: 60.0 FPS" instead of "FPS: 3"
• Show frame time breakdown (publish/upload/present)
• Show GPU adapter name
• Badge: "GPU Accelerated" in green

Files to modify

• `src-tauri/src/renderer/gpu.rs` — add metrics struct + getter
• `src-tauri/src/commands/session.rs` — add `get_gpu_metrics` command
• `src/components/session/PerformanceHUD.tsx` — display GPU metrics
• `src/hooks/useRdpSession.ts` — poll GPU metrics

Priority: P2 — observability for the GPU pipeline