refactor(#173): widget NineSliceAsset réutilisable (cartes + bandeau)

Presets cards/bandeau, cache images, appliqué au sélecteur couple.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/widgets/quotidien/cartes_feed.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import 'package:google_fonts/google_fonts.dart';
-import 'package:p_tits_pas/models/card_assets.dart';
 import 'package:p_tits_pas/models/carte_bulle.dart';
 import 'package:p_tits_pas/services/api/cards_service.dart';
 import 'package:p_tits_pas/widgets/quotidien/nine_slice_asset.dart';
@@ -288,11 +287,8 @@ class _BulleTile extends StatelessWidget {
     final treated = card.statut == CarteStatut.traitee;
     return Opacity(
       opacity: treated ? 0.55 : 1,
-      child: NineSliceAsset(
+      child: NineSliceAsset.card(
         assetPath: card.backgroundAsset,
-        centerSlice: CardColorHorizontal.centerSlice,
-        cornerWidth: CardColorHorizontal.cornerWidth,
-        cornerHeight: CardColorHorizontal.cornerHeight,
         constraints: const BoxConstraints(minHeight: 96),
         padding: const EdgeInsets.fromLTRB(20, 14, 20, 14),
         child: Column(