fix: add mobile hamburger menu, nav links were unreachable on mobile

Nav links were `hidden md:flex` with no mobile fallback — sub-768px
viewports showed only logo + avatar, no way to reach Explore, Feed,
Collections, etc. Add a Sheet-based hamburger menu with the same links.

apps/web/messages/en.json

@@ -1,5 +1,6 @@
 {
   "nav": {
+    "menu": "Menu",
     "recipes": "Recipes",
     "explore": "Explore",
     "search": "Search",

apps/web/messages/fr.json

@@ -1,5 +1,6 @@
 {
   "nav": {
+    "menu": "Menu",
     "recipes": "Recettes",
     "explore": "Explorer",
     "search": "Rechercher",