feat: CUDA backend swap via binary download and restart

Add the ability to download a CUDA-enabled backend binary (~2.4 GB) and
swap it in via a backend-only restart, solving the #1 user pain point
(19 open 'GPU not detected' issues caused by GitHub's 2 GB asset limit).

Backend:
- cuda_download.py: download from R2 (primary) or GitHub split-parts
  (fallback), SHA-256 verification, atomic writes, progress via SSE
- 4 new endpoints: GET/POST/DELETE /backend/cuda-*, GET cuda-progress
- server.py: --version flag, auto-detect variant from binary name
- build_binary.py: --cuda flag for CUDA PyInstaller builds
- split_binary.py: split large binaries into <2GB GitHub Release assets
- CI workflow for building CUDA binary

Tauri:
- restart_server command (stop -> wait -> start)
- start_server prefers CUDA binary from {data_dir}/backends/ if present
- Version mismatch check: runs --version before launching CUDA binary

Frontend:
- GpuAcceleration component: download, progress, restart, switch, delete
- API client + types for CUDA status and management
- Platform lifecycle: restartServer() on Tauri/Web
- Aggressive 1s health polling during restart for fast reconnection

web/src/platform/lifecycle.ts

@@ -15,6 +15,11 @@ class WebLifecycle implements PlatformLifecycle {
     // No-op for web - server is managed externally
   }
 
+  async restartServer(): Promise<string> {
+    // No-op for web - server is managed externally
+    return import.meta.env.VITE_SERVER_URL || 'http://localhost:17493';
+  }
+
   async setKeepServerRunning(_keep: boolean): Promise<void> {
     // No-op for web
   }