Add reset-to-default button and how-to docs to system prompt editor

Each service prompt card now shows:
- A collapsible how-to panel with placeholder docs, required JSON
  response keys, and usage notes
- A "Reset to Default" button (with confirmation step) that restores
  the built-in prompt without saving, letting the admin review first
- A "Using the built-in default prompt" indicator when unchanged

Backend includes default_system / default_user_template in the
system-prompts API response so the frontend never duplicates defaults.

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

backend/app/core/app_config.py

@@ -179,7 +179,7 @@ SYSTEM_PROMPT_SERVICES: dict[str, str] = {
 
 
 def load_all_system_prompts() -> dict:
-    """Return {service_id: {label, system, user_template}} for all registered services."""
+    """Return {service_id: {label, system, user_template, default_system, default_user_template}}."""
     result: dict = {}
     for service_id, label in SYSTEM_PROMPT_SERVICES.items():
         config = load_service_config(service_id)
@@ -189,6 +189,8 @@ def load_all_system_prompts() -> dict:
             "label": label,
             "system": prompts.get("system", defaults["system"]),
             "user_template": prompts.get("user_template", defaults["user_template"]),
+            "default_system": defaults["system"],
+            "default_user_template": defaults["user_template"],
         }
     return result