fix: CalendarPage API calls use relative URLs (not localhost:8000)

All fetch() calls now use relative paths (/api/eco/...) so nginx
can proxy them to the backend container in production.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/CalendarPage.tsx

@@ -77,7 +77,7 @@ const SORT_OPTIONS = [
   { id: 'series', label: 'Série' },
 ]
 
-const API_BASE = 'http://localhost:8000'
+const API_BASE = ''
 
 // ── Helpers ───────────────────────────────────────────────────────────────────