Enable CUDA for Windows/Linux builds + clean up old sidecars

- Windows and Linux sidecar builds now use --with-cuda for GPU acceleration
  (macOS stays CPU-only — Apple Silicon uses Metal, not CUDA)
- Windows upload switched from --data-binary to -T streaming for 2GB+ files
- Add cleanup_old_sidecars() that removes stale sidecar-* directories on
  startup, keeping only the current version
- Add NSIS uninstall hook to remove sidecar data dir on Windows uninstall
  (user data in ~/.voicetonotes is preserved)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -51,6 +51,9 @@
       }
     },
     "windows": {
+      "nsis": {
+        "installerHooks": "nsis-hooks.nsh"
+      },
       "wix": {
         "language": "en-US"
       }