feat: upload drag-drop + responsive mobile

Backend:
- Route POST /api/upload avec multipart (axum 0.8)
- Proxy vers Imago, validation MIME, feedback JSON

Frontend:
- Zone de drop avec drag & drop (effet visuel au survol)
- Bouton Upload dans le header + FAB flottant mobile
- Support Ctrl+V (coller depuis presse-papier)
- Upload multiple, feedback toast (succès/erreur)
- Raccourci Ctrl+U pour upload
- Responsive: 2 breakpoints (768px, 400px)
- Gallery 2 colonnes sur mobile, FAB, toasts bottom
- user-scalable=no, tap-highlight transparent

Cargo.toml

@@ -35,7 +35,7 @@ thiserror = "2"
 open = "5"
 
 # Web server (axum + tower)
-axum = { version = "0.8", features = ["ws"] }
+axum = { version = "0.8", features = ["ws", "multipart"] }
 tower = "0.5"
 tower-http = { version = "0.6", features = ["cors", "fs", "trace"] }
 # Frontend embedding