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/flask-api/ai_agent.py

@@ -9,7 +9,7 @@ import json
 import os
 import logging
 from openai import OpenAI, BadRequestError
-from legal_framework import LEGAL_FILTER_PROMPT, SYNTHESIS_PROMPT
+from legal_framework import LEGAL_FILTER_PROMPT, SYNTHESIS_PROMPT, LAW_CHECK_PROMPT
 
 logger = logging.getLogger(__name__)
 
@@ -107,6 +107,29 @@ def filter_idea(content: str) -> dict:
         return {"accepted": False, "reason": "Service temporairement indisponible"}
 
 
+def check_existing_law(content: str) -> str:
+    """
+    Vérifie si une proposition citoyenne est déjà couverte par une loi
+    française ou un règlement européen en vigueur.
+    Retourne une phrase informative (non bloquante).
+    """
+    try:
+        client = get_client()
+        model = os.environ.get("FILTER_MODEL", os.environ.get("OPENAI_FILTER_MODEL", "mistral-small-latest"))
+        response = client.chat.completions.create(
+            model=model,
+            max_tokens=200,
+            messages=[
+                {"role": "system", "content": LAW_CHECK_PROMPT},
+                {"role": "user", "content": f'Proposition : "{content}"'},
+            ],
+        )
+        return (response.choices[0].message.content or "").strip()
+    except Exception:
+        logger.exception("Erreur lors de la vérification du cadre légal existant")
+        return ""
+
+
 def synthesize_ideas(ideas: list[str]) -> str:
     """
     Synthétise une liste d'idées acceptées en un texte collectif