feat: CI/CD workflow for automated MSI/AppImage/deb desktop build

- .gitea/workflows/desktop-build.yml: compiles on push to main
- Windows runner: Rust + Tauri + Python embed → MSI
- Linux runner: Rust + Tauri + webkit2gtk → AppImage + .deb
- Artifacts uploaded with 30-day retention
- Triggers when desktop/, frontend/, or backend/ changes
- Manual trigger via workflow_dispatch
- Updated ROADMAP #77 to mark CI/CD as done

docs/ROADMAP.md

@@ -731,6 +731,7 @@
   - [ ] **Raccourcis clavier** : `Ctrl+N` nouveau fichier, `Ctrl+O` ouvrir vault, `Ctrl+,` paramètres
 
 ##### D. Build et distribution (2 jours)
+  - [x] **CI/CD automatisé** : workflow Gitea Actions `.gitea/workflows/desktop-build.yml` — build Windows + Linux à chaque push sur `main` (si `desktop/` modifié), upload des artefacts `.msi`/`.AppImage`/`.deb` en release
   - [ ] **Build Windows** : `tauri build --target x86_64-pc-windows-msvc` → `.msi` + `.exe` installer
   - [ ] **Build Linux** : `tauri build --target x86_64-unknown-linux-gnu` → `.deb`, `.rpm`, `.AppImage`
   - [ ] **Auto-update** : endpoint `GET /api/desktop/update` → compare version → télécharge nouveau `.msi`