feat(nav): bouton action intégré dans la navbar v0.4.8

- ActionButtonContext : contexte React permettant aux pages d'injecter
  leur bouton action dans la navbar (Shopping=fa-cart-plus, Todos/Notes=+)
- BottomNav : 5e slot dédié avec dock circulaire visuel permanent ;
  bouton rendu 10px au-dessus du centre du slot (effet soulevé)
- Layout : ActionButtonProvider + overflow visible sur le conteneur nav
- Pages : useEffect enregistre/vide le bouton action — plus de FAB flottant
  sur le contenu, liste entièrement visible

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

frontend/src/pages/ShoppingPage.tsx

@@ -1,5 +1,5 @@
 // frontend/src/pages/ShoppingPage.tsx
-import { useState, useEffect, useCallback } from 'react'
+import { useState, useEffect, useCallback, useRef } from 'react'
 import { matchesSearch } from '../utils/search'
 import type { ShoppingListDetail, ShoppingList, Store, Product, ShoppingItem } from '../api/shopping'
 import {
@@ -13,6 +13,7 @@ import ItemRow from '../components/shopping/ItemRow'
 import CatalogueModal from '../components/shopping/CatalogueModal'
 import BoutiquesModal from '../components/shopping/BoutiquesModal'
 import { useWakeLock } from '../hooks/useWakeLock'
+import { useActionButton } from '../contexts/ActionButtonContext'
 
 const inputStyle: React.CSSProperties = {
   width: '100%',
@@ -76,6 +77,33 @@ export default function ShoppingPage() {
 
   useWakeLock(currentList !== null)
 
+  const { setActionButton } = useActionButton()
+  const openAddSheetRef = useRef(openAddSheet)
+  useEffect(() => { openAddSheetRef.current = openAddSheet })
+
+  useEffect(() => {
+    if (!currentList || showAddSheet) {
+      setActionButton(null)
+      return
+    }
+    setActionButton(
+      <button
+        onClick={() => openAddSheetRef.current()}
+        aria-label="Ajouter un article"
+        style={{
+          width: 56, height: 56, borderRadius: '50%',
+          background: 'var(--accent)', color: '#1d2021', border: 'none',
+          fontSize: 22, cursor: 'pointer',
+          boxShadow: '0 4px 16px rgba(0,0,0,0.5)',
+          display: 'flex', alignItems: 'center', justifyContent: 'center',
+        }}
+      >
+        <i className="fa-solid fa-cart-plus" />
+      </button>
+    )
+    return () => setActionButton(null)
+  }, [currentList, showAddSheet, setActionButton])
+
   const loadData = useCallback(async () => {
     setLoading(true)
     setError(null)
@@ -526,22 +554,6 @@ export default function ShoppingPage() {
             )}
           </div>
 
-          {/* FAB — masqué quand le sheet est ouvert */}
-          {!showAddSheet && (
-            <button
-              onClick={openAddSheet}
-              aria-label="Ajouter un article"
-              style={{
-                position: 'fixed', bottom: 10, right: 12,
-                width: 56, height: 56, borderRadius: '50%',
-                background: 'var(--accent)', color: '#1d2021', border: 'none',
-                fontSize: 22, cursor: 'pointer', boxShadow: '0 4px 12px rgba(0,0,0,0.5)',
-                display: 'flex', alignItems: 'center', justifyContent: 'center',
-              }}
-            >
-              <i className="fa-solid fa-cart-plus" />
-            </button>
-          )}
         </>
       )}