feat: Implement LLM provider deletion and refactor ConfigManager to preserve unknown payload sections.

frontend/src/lib/i18n/locales/en.json

@@ -111,6 +111,9 @@
     "connection_success": "Connection successful!",
     "connection_failed": "Connection failed: {error}",
     "no_providers": "No providers configured.",
+    "delete_confirm": "Delete LLM provider \"{name}\"?",
+    "delete_success": "Provider \"{name}\" deleted.",
+    "delete_failed": "Failed to delete provider: {error}",
     "multimodal": "Multimodal",
     "text_only": "Text only",
     "doc_preview_title": "Documentation Preview",