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/src/sidecar/mod.rs

@@ -68,15 +68,15 @@ impl SidecarManager {
         };
 
         // Versioned extraction directory prevents stale sidecar after app updates
-        let extract_dir = DATA_DIR
-            .get()
-            .ok_or("App data directory not initialized")?
-            .join(format!("sidecar-{}", env!("CARGO_PKG_VERSION")));
+        let data_dir = DATA_DIR.get().ok_or("App data directory not initialized")?;
+        let current_version = env!("CARGO_PKG_VERSION");
+        let extract_dir = data_dir.join(format!("sidecar-{}", current_version));
 
         let binary_path = extract_dir.join(binary_name);
 
         // Already extracted — use it directly
         if binary_path.exists() {
+            Self::cleanup_old_sidecars(data_dir, current_version);
             return Ok(binary_path);
         }
 
@@ -102,6 +102,7 @@ impl SidecarManager {
             }
         }
 
+        Self::cleanup_old_sidecars(data_dir, current_version);
         Ok(binary_path)
     }
 
@@ -182,6 +183,44 @@ impl SidecarManager {
         Ok(())
     }
 
+    /// Remove old sidecar-* directories that don't match the current version.
+    /// Called after the current version's sidecar is confirmed ready.
+    fn cleanup_old_sidecars(data_dir: &Path, current_version: &str) {
+        let current_dir_name = format!("sidecar-{}", current_version);
+
+        let entries = match std::fs::read_dir(data_dir) {
+            Ok(entries) => entries,
+            Err(e) => {
+                eprintln!("[sidecar-rs] Cannot read data dir for cleanup: {e}");
+                return;
+            }
+        };
+
+        for entry in entries.flatten() {
+            let name = entry.file_name();
+            let name_str = name.to_string_lossy();
+
+            if !name_str.starts_with("sidecar-") {
+                continue;
+            }
+            if *name_str == current_dir_name {
+                continue;
+            }
+            if entry.path().is_dir() {
+                eprintln!(
+                    "[sidecar-rs] Removing old sidecar: {}",
+                    entry.path().display()
+                );
+                if let Err(e) = std::fs::remove_dir_all(entry.path()) {
+                    eprintln!(
+                        "[sidecar-rs] Failed to remove {}: {e}",
+                        entry.path().display()
+                    );
+                }
+            }
+        }
+    }
+
     /// Find a working Python command for the current platform.
     fn find_python_command() -> &'static str {
         if cfg!(target_os = "windows") {