fix(service-party-app): fix participants display in Home page

listShares returned `participants` but Home.tsx expected `metadata.participants`

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-app/electron/main.ts

@@ -1262,7 +1262,9 @@ function setupIpcHandlers() {
         publicKey: share.public_key_hex,
         createdAt: share.created_at,
         lastUsedAt: share.last_used_at,
-        participants: JSON.parse(share.participants_json || '[]'),
+        metadata: {
+          participants: JSON.parse(share.participants_json || '[]'),
+        },
       }));
       return { success: true, data: formatted };
     } catch (error) {