feat: navigation swipe mobile horizontale

- App.svelte : swipe gauche/droite → même comportement que les touches clavier (navigation entre slides)
- Play.svelte : stopImmediatePropagation sur touchend, blocage du scroll vertical (touchmove)
- Portfolio.svelte : migration du touch vertical (composable) vers horizontal — navigation entre projets, slide voisine aux bords, blocage scroll vertical, debounce 650ms anti-spam

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.svelte

@@ -79,9 +79,45 @@
       }
     }
     window.addEventListener('keydown', handleKeydown)
+
+    // Swipe gauche/droite → même comportement que les touches clavier
+    const TOUCH_THRESHOLD = 50
+    let touchStartX = 0
+    let touchStartY = 0
+
+    const handleTouchStart = (e) => {
+      touchStartX = e.touches[0].clientX
+      touchStartY = e.touches[0].clientY
+    }
+
+    const handleTouchEnd = (e) => {
+      const deltaX = touchStartX - e.changedTouches[0].clientX
+      const deltaY = touchStartY - e.changedTouches[0].clientY
+      if (Math.abs(deltaX) < TOUCH_THRESHOLD) return
+      if (Math.abs(deltaY) > Math.abs(deltaX)) return
+
+      const activePath = slides.active?.path ?? ''
+      const currentPath = window.location.pathname.replace(/^\/en/, '') || '/'
+      const isSubPage = activePath && currentPath.startsWith(activePath + '/')
+      if (isSubPage) return
+
+      if (deltaX > 0) {
+        const next = slides.all[slides.activeIndex + 1]
+        if (next) slideTo(next.path)
+      } else {
+        const prev = slides.all[slides.activeIndex - 1]
+        if (prev) slideTo(prev.path)
+      }
+    }
+
+    window.addEventListener('touchstart', handleTouchStart, { passive: true })
+    window.addEventListener('touchend', handleTouchEnd, { passive: true })
+
     return () => {
       window.removeEventListener('resize', handleResize)
       window.removeEventListener('keydown', handleKeydown)
+      window.removeEventListener('touchstart', handleTouchStart)
+      window.removeEventListener('touchend', handleTouchEnd)
       clearTimeout(resizeTimer)
     }
   })