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>

frontend/css/style.css

@@ -109,12 +109,12 @@ html, body {
   position: absolute;
   bottom: 12px;
   right: 12px;
-  width: 40px;
-  height: 40px;
+  width: 56px;
+  height: 56px;
   display: flex;
   align-items: center;
   justify-content: center;
-  font-size: 1.2rem;
+  font-size: 1.5rem;
   color: rgba(255,255,255,0.15);
   cursor: pointer;
   border-radius: 50%;
@@ -381,12 +381,13 @@ html, body {
   background: var(--fond-carte);
   border: 2px solid transparent;
   border-radius: 12px;
-  padding: 0.6rem 1.2rem;
+  padding: 1rem 1.5rem;
   color: var(--texte);
-  font-size: 0.9rem;
+  font-size: 1.1rem;
   cursor: pointer;
   transition: var(--transition);
   white-space: nowrap;
+  min-height: 48px;
 }
 
 .btn-filtre.actif, .btn-overlay.actif {
@@ -467,8 +468,8 @@ html, body {
 }
 
 .btn-exemplaire {
-  width: 48px;
-  height: 48px;
+  width: 56px;
+  height: 56px;
   border-radius: 50%;
   border: 2px solid var(--primaire);
   background: transparent;
@@ -614,9 +615,10 @@ html, body {
   color: var(--texte-secondaire);
   border: 2px solid var(--texte-secondaire);
   border-radius: 12px;
-  padding: 0.6rem 1.5rem;
-  font-size: 1rem;
+  padding: 1rem 2rem;
+  font-size: 1.1rem;
   cursor: pointer;
+  min-height: 48px;
 }
 
 .btn-fermer {
@@ -625,7 +627,9 @@ html, body {
   color: var(--texte);
   font-size: 2rem;
   cursor: pointer;
-  padding: 0.5rem;
+  padding: 1rem;
+  min-width: 48px;
+  min-height: 48px;
 }
 
 .btn-danger {