fix: résoudre python-embed relativement à l'exe + copier runtime dans output dir

- main.rs: utiliser current_exe().parent() au lieu de ./ relatif au cwd
- build-windows.bat: étape 5b qui copie python-embed/ et backend/ dans target/release

desktop/src/main.rs

@@ -10,10 +10,17 @@ struct BackendProcess(Mutex<Option<Child>>);
 // ── Sidecar : spawn Python backend ──────────────────────────────────────────
 
 fn spawn_backend() -> Result<Child, String> {
+    // Resolve python-embed relative to the executable, not cwd
+    let exe_dir = std::env::current_exe()
+        .map_err(|e| format!("Cannot get exe path: {}", e))?
+        .parent()
+        .ok_or("Cannot get exe parent dir")?
+        .to_path_buf();
+
     let python = if cfg!(target_os = "windows") {
-        "./python-embed/python.exe"
+        exe_dir.join("python-embed").join("python.exe")
     } else {
-        "./python-embed/bin/python3"
+        exe_dir.join("python-embed").join("bin").join("python3")
     };
 
     let child = Command::new(python)