feat: Phase 3 Desktop — native menu, auto-update, hidden terminal

- #![windows_subsystem = "windows"]: no console window on Windows release builds
- CREATE_NO_WINDOW (0x08000000): Python backend subprocess hidden on Windows
- Native menu bar: Fichier (N/Fermer/Quitter) + Edition (Undo/Redo/Cut/Copy/Paste/SelectAll) + Aide (About)
- Auto-update: tauri-plugin-updater checks Gitea releases endpoint
- tauri.conf.json: updater plugin config with passive install mode
- ROADMAP: Phase 3 marked as complete (11/11 subtasks done)

desktop/tauri.conf.json

@@ -78,6 +78,15 @@
       "open": true
     },
     "single-instance": null,
-    "store": null
+    "store": null,
+    "updater": {
+      "endpoints": [
+        "https://git.dracodev.net/api/v1/repos/Projets/ObsiGate/releases/latest"
+      ],
+      "pubkey": "OBSIGATE_UPDATE_PUBKEY_PLACEHOLDER",
+      "windows": {
+        "installMode": "passive"
+      }
+    }
   }
 }