feat: add CSV export for company analysis results

Add GET /export/{company_name} backend endpoint that returns analysis
records as a downloadable CSV file. Add Export CSV button to the
Analysis page that triggers the download via the API.

Closes leeworks-agents/SPARC#20

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -126,6 +126,23 @@ export const analysisApi = {
   },
 };
 
+// Export API
+export const exportApi = {
+  exportCsv: async (companyName: string): Promise<void> => {
+    const response = await api.get(`/export/${encodeURIComponent(companyName)}`, {
+      responseType: 'blob',
+    });
+    const url = window.URL.createObjectURL(new Blob([response.data]));
+    const link = document.createElement('a');
+    link.href = url;
+    link.setAttribute('download', `sparc_${companyName.toLowerCase().replace(/\s+/g, '_')}_export.csv`);
+    document.body.appendChild(link);
+    link.click();
+    link.remove();
+    window.URL.revokeObjectURL(url);
+  },
+};
+
 // Analytics API
 export const analyticsApi = {
   getAnalytics: async (days = 30): Promise<Analytics> => {