build(cuda): ship a CUDA installer variant with bundled runtime DLLs

The default build (default = candle-cuda) load-time-imports the CUDA runtime,
so it crashes on any machine without the toolkit. Add a CUDA release variant
that bundles the redistributable runtime DLLs (cudart/cublas/cublasLt/curand,
verified via dumpbin) so it runs on any NVIDIA machine with just a driver.

- tauri.cuda.conf.json: a 'tauri build --config' overlay (CPU release
  untouched) that points the updater at latest-cuda.json so a CUDA install
  never self-updates into the CPU build
- windows/cuda-hooks.nsh: NSIS hook embeds the DLLs via File and extracts them
  next to phokus.exe, where the Windows loader searches
- build:app:cuda script; cuda-redist/ gitignored (DLLs copied from the toolkit
  locally), with a negation so the overlay stays tracked

package.json

@@ -11,6 +11,7 @@
     "dev:app": "tauri dev",
     "dev:app:cpu": "tauri dev -- --no-default-features",
     "build:app:cpu": "tauri build -- --no-default-features",
+    "build:app:cuda": "tauri build --config src-tauri/tauri.cuda.conf.json",
     "dev:vite": "vite",
     "preview": "vite preview",
     "tauri": "tauri"