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/nsis-hooks.nsh

@@ -0,0 +1,11 @@
+; NSIS uninstall hook for Voice to Notes
+; Removes the sidecar data directory (extracted sidecar binaries + logs)
+; but preserves user data in $PROFILE\.voicetonotes (database, settings, models)
+
+!macro NSIS_HOOK_POSTUNINSTALL
+  ; Remove the Tauri app_local_data_dir which contains:
+  ;   - Extracted sidecar directories (voice-to-notes-sidecar/)
+  ;   - sidecar.log
+  ; Path: %LOCALAPPDATA%\com.voicetonotes.app
+  RMDir /r "$LOCALAPPDATA\com.voicetonotes.app"
+!macroend