feat: mode application de bureau + packaging Windows autonome

Permet de distribuer l'application en executable Windows double-clic,
sans Python/Node/terminal pour l'utilisateur final.

- `desktop.py` : demarre le serveur sur un port libre dans un thread et
  ouvre une fenetre native (pywebview). `PLESNA_DEBUG=1` active les
  DevTools. Commande `plesna-gerance desktop` (import paresseux).
- `paths.py` : resolution centralisee des chemins selon le contexte
  (dev, executable PyInstaller, surcharge par variables d'env). Les
  donnees vont dans un dossier utilisateur inscriptible (%APPDATA%),
  les ressources dans le bundle. connection.py/storage.py utilisent
  `get_data_dir()`, app.py `resource_path()`.
- app.py : type MIME `.mjs` force (modules ES / worker PDF.js sous
  Windows).
- Groupes de deps optionnels desktop / desktop-linux / build.
- packaging/ : spec PyInstaller, build_windows.ps1, installeur Inno
  Setup. Workflow GitHub build-windows.yml (runner Windows).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plesna_gerance/database/connection.py

@@ -7,33 +7,23 @@ from pathlib import Path
 from sqlalchemy import create_engine, text
 from sqlalchemy.orm import Session, sessionmaker
 
+from ..paths import get_data_dir
 from .models import Base, Tag
 
-
-# Default database path - relative to project root
-def _get_project_root() -> Path:
-    """Find project root by looking for pyproject.toml."""
-    current = Path(__file__).resolve()
-    for parent in current.parents:
-        if (parent / "pyproject.toml").exists():
-            return parent
-    # Fallback to home directory if not found
-    return Path.home() / ".plesna_gerance"
-
-
-DEFAULT_DB_PATH = _get_project_root() / "data" / "database.sqlite"
-
 # Global engine instance
 _engine = None
 _SessionLocal = None
 
 
 def get_db_path() -> Path:
-    """Get database path from environment or default."""
+    """Get database path from environment or default.
+
+    Resolved lazily so a packaged build writes to the per-user data dir.
+    """
     env_path = os.environ.get("PLESNA_DB_PATH")
     if env_path:
         return Path(env_path)
-    return DEFAULT_DB_PATH
+    return get_data_dir() / "database.sqlite"
 
 
 def _build_engine(db_path: Path):