fix: test connection always showed success; add per-account test endpoint

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/6268035d-325f-422a-886f-8b2919127261

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/src/lib/api.ts

@@ -338,6 +338,13 @@ export const mailAccountsApi = {
     return response.data;
   },
 
+  async testExisting(accountId: number): Promise<{ success: boolean; message: string }> {
+    const response = await api.post<{ success: boolean; message: string }>(
+      `/mail-accounts/${accountId}/test`
+    );
+    return response.data;
+  },
+
   async autoDetect(
     emailAddress: string
   ): Promise<{ success: boolean; suggestions: AutoDetectSuggestion[] }> {