Support webcam (OpenCV) + optimisation tactile

- Camera: support webcam v4l2 via OpenCV en plus de gphoto2
- API: selection de source camera (webcam:0, gphoto2, etc.)
- CSS: boutons agrandis pour usage tactile (min 48px)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -103,15 +103,17 @@ async def api_preview():
 async def api_camera_statut():
     return {
         "connectee": camera.connectee,
+        "mode": camera.mode,
         "appareils": camera.lister_appareils(),
     }
 
 
 @app.post("/api/camera/reconnecter")
-async def api_camera_reconnecter():
+async def api_camera_reconnecter(body: dict = {}):
     camera.deconnecter()
-    ok = camera.connecter()
-    return {"connectee": ok}
+    source = body.get("source")
+    ok = camera.connecter(source=source)
+    return {"connectee": ok, "mode": camera.mode}
 
 
 # --- API Effets ---