fix(sign): use threshold_n from API response instead of parties.length

The validateSigningSession handler was using parties.length for threshold.n
which returned 0 when parties array was empty. Now correctly uses the
threshold_n value returned from the backend API.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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

@@ -1407,11 +1407,12 @@ function setupIpcHandlers() {
           messageHash: result?.message_hash,
           threshold: {
             t: result?.threshold_t,
-            n: result?.parties?.length || 0,
+            n: result?.threshold_n,
           },
           currentParticipants: result?.joined_count || 0,
           status: result?.status,
           parties: result?.parties,
+          initiator: '发起者',
         },
       };
     } catch (error) {