Add mobile overflow nav, law check feature, and favicon

Nav: "Fonctionnement" and "Flyer QR" restored on mobile via a ···
overflow dropdown (click-outside closes); other links always visible

Law check: new POST /api/check-law endpoint (5/min, 20/h) calls
mistral-small with a new LAW_CHECK_PROMPT to detect if a proposal is
already covered by French or EU law. Informational only, non-blocking.
Frontend: "Vérifier le cadre légal existant" button below textarea in
home.tsx; result displayed with Scale icon; resets on content change.

Favicon: replaced placeholder red square with a petrol rounded square
containing a serif "V" (for Voix) in warm cream — matches brand palette

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

artifacts/voix-du-peuple/public/favicon.svg

@@ -1,3 +1,15 @@
-<svg width="180" height="180" viewBox="0 0 180 180" fill="none" xmlns="http://www.w3.org/2000/svg">
-<rect width="180" height="180" rx="36" fill="#FF3C00"/>
+<svg width="32" height="32" viewBox="0 0 32 32" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <!-- Fond arrondi couleur primaire petrol -->
+  <rect width="32" height="32" rx="7" fill="#29676F"/>
+  <!-- Lettre V stylisée centrée, police serif grasse -->
+  <text
+    x="16" y="24"
+    font-family="Georgia, 'Times New Roman', serif"
+    font-size="22"
+    font-weight="700"
+    fill="#F9F7F1"
+    text-anchor="middle"
+    dominant-baseline="auto"
+    letter-spacing="-1"
+  >V</text>
 </svg>