feat: Tauri desktop shell — Rust backend + config files + build scripts

- desktop/Cargo.toml: Tauri + plugins (dialog, notification, fs, updater)
- desktop/tauri.conf.json: Windows (.msi/.nsis) + Linux (.deb/.AppImage) config
- desktop/src/main.rs: spawn Python backend, health check, tray icon, commands
- desktop/build.rs: Tauri build hook
- desktop/python-embed/sidecar.py: uvicorn launcher with port auto-detect
- build-windows.bat: automated Windows build with Python embed download
- build-linux.sh: automated Linux build with venv
- desktop/README.md: architecture doc + usage guide
- desktop/validate-structure.sh: CI validation script
- ROADMAP.md: #77 Tauri Desktop section added

desktop/src/main.rs

@@ -0,0 +1,149 @@
+use tauri::Manager;
+use std::process::{Child, Command};
+use std::sync::Mutex;
+use std::time::Duration;
+
+// ── State ───────────────────────────────────────────────────────────────────
+
+struct BackendProcess(Mutex<Option<Child>>);
+
+// ── Sidecar : spawn Python backend ──────────────────────────────────────────
+
+fn spawn_backend() -> Result<Child, String> {
+    let python = if cfg!(target_os = "windows") {
+        "./python-embed/python.exe"
+    } else {
+        "./python-embed/bin/python3"
+    };
+
+    let child = Command::new(python)
+        .args([
+            "-m", "uvicorn",
+            "backend.main:app",
+            "--host", "127.0.0.1",
+            "--port", "17890",
+            "--log-level", "info",
+        ])
+        .spawn()
+        .map_err(|e| format!("Failed to start Python backend: {}", e))?;
+
+    Ok(child)
+}
+
+// ── Health check — wait until backend is ready ──────────────────────────────
+
+async fn wait_for_backend() -> Result<(), String> {
+    let client = reqwest::Client::new();
+    let url = "http://127.0.0.1:17890/api/health";
+
+    for i in 0..30 {
+        match client.get(url).timeout(Duration::from_secs(2)).send().await {
+            Ok(resp) if resp.status().is_success() => {
+                println!("✅ Backend ready (attempt {})", i + 1);
+                return Ok(());
+            }
+            _ => {
+                tokio::time::sleep(Duration::from_millis(500)).await;
+            }
+        }
+    }
+
+    Err("Backend did not become ready within 15 seconds".to_string())
+}
+
+// ── Tauri Commands ──────────────────────────────────────────────────────────
+
+#[tauri::command]
+fn get_backend_url() -> String {
+    "http://127.0.0.1:17890".to_string()
+}
+
+#[tauri::command]
+async fn pick_vault_folder(app: tauri::AppHandle) -> Result<String, String> {
+    use tauri_plugin_dialog::DialogExt;
+
+    let path = app
+        .dialog()
+        .file()
+        .blocking_pick_folder();
+
+    match path {
+        Some(p) => Ok(p.to_string()),
+        None => Err("No folder selected".to_string()),
+    }
+}
+
+#[tauri::command]
+fn get_system_theme(app: tauri::AppHandle) -> String {
+    #[cfg(any(target_os = "windows", target_os = "linux", target_os = "macos"))]
+    {
+        use tauri::Theme;
+        match app.get_webview_window("main").unwrap().theme() {
+            Ok(Theme::Dark) => "dark".to_string(),
+            _ => "light".to_string(),
+        }
+    }
+}
+
+// ── Entry Point ─────────────────────────────────────────────────────────────
+
+fn main() {
+    // 1. Start Python backend
+    println!("🚀 Starting ObsiGate backend...");
+    let backend = spawn_backend().expect("Failed to launch Python backend");
+    let backend_state = BackendProcess(Mutex::new(Some(backend)));
+
+    // 2. Build Tauri app
+    tauri::Builder::default()
+        .plugin(tauri_plugin_shell::init())
+        .plugin(tauri_plugin_dialog::init())
+        .plugin(tauri_plugin_notification::init())
+        .plugin(tauri_plugin_fs::init())
+        .plugin(tauri_plugin_process::init())
+        .manage(backend_state)
+        .invoke_handler(tauri::generate_handler![
+            get_backend_url,
+            pick_vault_folder,
+            get_system_theme,
+        ])
+        .setup(|app| {
+            // 3. Wait for backend health check (non-blocking)
+            let handle = app.handle().clone();
+            tauri::async_runtime::spawn(async move {
+                match wait_for_backend().await {
+                    Ok(()) => {
+                        // Navigate to the main app
+                        if let Some(window) = handle.get_webview_window("main") {
+                            let _ = window.eval("window.location.href = 'http://127.0.0.1:17890'");
+                        }
+                    }
+                    Err(e) => {
+                        eprintln!("❌ Backend failed: {}", e);
+                        if let Some(window) = handle.get_webview_window("main") {
+                            let _ = window.eval(&format!(
+                                "document.body.innerHTML = '<div style=\"padding:40px;text-align:center;font-family:sans-serif;color:red\"><h2>ObsiGate n\\'a pas pu démarrer</h2><p>{}</p><button onclick=\"location.reload()\">Réessayer</button></div>'",
+                                e.replace('\'', "\\'")
+                            ));
+                        }
+                    }
+                }
+            });
+
+            Ok(())
+        })
+        .on_window_event(|window, event| {
+            if let tauri::WindowEvent::CloseRequested { .. } = event {
+                // Kill the Python backend on close
+                let state = window.state::<BackendProcess>();
+                if let Ok(mut guard) = state.0.lock() {
+                    if let Some(ref mut child) = *guard {
+                        println!("🛑 Stopping backend...");
+                        let _ = child.kill();
+                        let _ = child.wait();
+                    }
+                }
+            }
+        })
+        .run(tauri::generate_context!())
+        .expect("error while running ObsiGate Desktop");
+}